Zoom lens

ABSTRACT

A zoom lens is disclosed, comprising, from front to rear, a first lens group of negative refractive power, a second lens group of positive refractive power and a third lens group of negative power, zooming being performed by varying a separation between each adjacent two of the lens groups, wherein for an object at infinity, an aspheric sphere of such shape that a negative refractive power gets progressively stronger toward the margin is applied to a lens surface of negative refracting power concave toward the object side which satisfies the following condition: 
     
         Hiw&gt;|Hbiw| 
    
     
         Hit&gt;|Hbit| 
    
     where Hiw (Hiw&gt;0) and Hit (Hit&gt;0) are the heights of incidence for the wide-angle end and the telephoto end of the on-axial ray of light on the i-th surface, respectively, and Hbiw and Hbit are the heights of incidence for the wide-angle end and the telephoto end of the off-axial principal ray of light from the maximum angle of field on the i-th surface, respectively.

This application is a continuation of application Ser. No. 08/348,816filed Nov. 28, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to zoom lenses suited to be used in photographiccameras for 35 mm film, video cameras or still video cameras and, moreparticularly, to wide-angle high range zoom lenses. Still moreparticularly, it relates to improvements of the compact form of theentire lens system along with high optical performance by setting forthappropriate rules of design for the construction and arrangement of theconstituent lenses in each of the lens groups and by applying asphericsurfaces of appropriate shapes to prescribed lens surfaces.

2. Description of the Related Art

It has been known to provide a zoom lens in which the leading lens groupis negative in the refractive power as one of the zoom types. Thisso-called "negative lead" type is relatively feasible for widening theangle of field and has found its use in many wide-angle zoom lenses ofnot less than 70° in the maximum angle of field.

For example, Japanese Patent Publication No. Sho 59-16248 proposes azoom lens having a first group of negative refractive power and a secondgroup of positive refractive power, totaling two lens groups, whereinzooming is performed by varying the separation between these two groups.This is usually called the "short" zoom lens.

Other Japanese Laid-Open Patent Applications Nos. Hei 2-72316 and Hei3-233422 propose 3-group wide-angle zoom lenses of which the firstgroup, when counted from the object side, is of negative refractivepower, the second group is of positive refractive power and the thirdgroup is of negative refractive power, wherein these groups are all mademovable for zooming.

Another Japanese Laid-Open Patent Application No. Hei 2-72316 proposes a4-group zoom lens of which the first group, when counted from the objectside, is of negative refractive power, the second group is of positiverefractive power, the third group is of positive refractive power andthe fourth group is of negative refractive power, wherein these fourgroups are all made movable for zooming.

Meanwhile, as another zoom type, there has been known a zoom lenscomprising, from front to rear, a first group of positive refractivepower and a second group of negative refractive power, totaling two lensgroups, both of which are made movable for zooming. Such a so-called2-group zoom type is feasible for minimizing the bulk and size of theentire lens system, so that it has been used in many leaf shuttercameras or like compact cameras. Since, in the 2-group zoom type, allthe function of varying the focal length is laid on only one lens group(that is, the second group), many zoom lenses of this type have theirranges limited to 1.6 to 2 or thereabout. If the 2-group type is appliedto form a zoom lens of too much increased range, the lens system suffersfrom a rapid increase of the size. Moreover, it becomes difficult tokeep the optical performance at a high level.

With the 2-group zoom lens, the first group may be divided into twoparts of positive refractive powers. The total number of lens groupsamounts to 3 and the plus-plus-minus refractive power arrangement isthus formed to aim at a great increase of the range. Such a 3-group zoomtype is proposed in, for example, Japanese Laid-Open Patent ApplicationsNos. Hei 3-73907, Hei 3-282409, Hei 4-37810 and Hei 4-76511.

When the 3-group type is applied to achieve a zoom lens of wider maximumsemi-angle of field than 35°, the entrance pupil is caused to vary itsposition to a larger extent with zooming. For the range to increase,therefore, it becomes difficult to suppress the variation of aberrationswith zooming to a minimum.

In general, zoom lenses of the negative lead type are relatively easilyamenable to widen the angle of field. However, to fulfill therequirements of increasing the maximum angle of field to more than 70°and of obtaining good optical image quality over the entire area of theimage frame at once, there is a need to find out appropriate rules ofdesign for the refractive power arrangement of all the lens groups andthe forms of the constituent members in the lens groups. If therefractive power arrangement of the lens groups and their forms areinappropriate, the variation of aberrations with zooming increases sogreatly that good stability of high optical performance is hardlymaintained throughout the entire zooming range, no matter how manyconstituent lenses may be used.

For a zoom lens to have not only wider angles of field, but also anincreased range, the use of aspheric surfaces is very effective toachieve minimization of the size of the entire lens system andimprovement of the optical performance. In this regard, however, to makea determination of what lens surfaces to select for introduction of anaspheric surface is very important, since it is depending on this thatthe aspheric surfaces produce greatly different effects of correctingaberrations. If the aspheric surface is not introduced into theappropriate lens surface, effective correction of aberrations becomesdifficult to do.

For example, the before-mentioned Japanese Laid-Open Patent ApplicationsNos. Hei 3-282409, Hei 4-37810 and Hei 4-76511 show that the positivelens in the second lens unit is selected to introduce an asphericsurface of such shape that the positive refractive power getsprogressively weaker as the distance from the optical axis increases. Inany of these zoom lenses, the selected one of the lens surface forintroduction of the aspheric surface takes its place at a considerableaxial distance from the stop so that the height of incidence of eitherthe on-axial ray or the off-axial ray on that selected lens surfacediffers with different focal lengths. This phenomenon is used to correctthe variation of aberrations. However, as the maximum angle of fieldwidens and the range also increases, the ranges of variation ofspherical aberration, for example, and of off-axial aberrations arecaused to increase greatly. So, it becomes difficult for this asphericsurface to correct these aberrations simultaneously.

U.S. Pat. No. 5,069,536 discloses a 3-group zoom lens of plus-plus-minuspower arrangement, wherein an aspheric surface is introduced into thatlens surface in the second lens unit of positive refractive power whichlies near the stop, is concave toward the object side and has a negativerefracting function. However, if, in this zoom lens, a widening of themaximum angle of field and a great increase of the range are to beachieved, the total zooming movement of each of the lens groupsincreases rapidly and also the front lens members in the first lensgroup get a large diameter. Thus, a long bulky lens system results.Moreover, as the refractive power of each of the lens groups isstrengthened, the range of variation of aberrations increases, which isdifficult to correct well by that aspheric surface.

Japanese Laid-Open Patent Application No. Hei 3-249614 shows awide-angle high range zoom lens comprising four lens groups ofplus-plus-plus-minus refractive power arrangement, or four lens groupsof plus-minus-plus-minus refractive power arrangement. However, thetotal number of constituent lenses is too large and the aspheric surfacedoes not effect sufficient results.

Japanese Laid-Open Patent Application No. Hei 3-73907 shows a zoom lenscomprising three lens groups of plus-plus-minus refractive powerarrangement, wherein at least that positive lens in the second lensgroup of positive refractive power which lies near the stop is selectedto introduce an aspheric surface of such shape that the positiverefractive power gets progressively weaker as the height from theoptical axis increases. With the help of this, a wide-angle zoom lens isachieved. However, the second group has a large number of lens elementsand the aspheric surface in the same lens group does not sufficientlycontribute to improvements of the compact form.

Japanese Laid-Open Patent Application No. Hei 3-233422 shows a zoom lenscomprising three lens groups of minus-plus-minus refractive powerarrangement, wherein a plurality of aspheric surfaces are used in thethird lens group. However, the optical total length is of the order of66 mm even at the wide-angle end. So, it can hardly be said that thetotal length of the whole lens system is short. Moreover. if a furtherwidening of the maximum angle of field is done, good correction ofaberrations is difficult to perform over the entire zooming range.

SUMMARY OF THE INVENTION

The invention is applied to the zoom lens having a plurality of lensgroups, for example, three in total, and sets forth rules for selectionof an appropriate lens group or lens surface to which an asphericsurface is applied. Thus, the aberrations which would come to be aproblem when the maximum angle of field widens and the zooming rangeincreases are corrected well. It is, therefore, an object of theinvention to provide a zoom lens which is improved in the compact formof the entire lens system, while still permitting maintenance of highoptical performance throughout the entire zooming range to be achieved.

Another object of the invention is to provide a wide-angle high rangezoom lens employing the "rear" focusing method. As the zooming movementsof the lens groups and the paraxial refractive power arrangements aremade appropriate, good stability of optical performance is maintainedagainst zooming from the wide-angle end to the telephoto end, orthroughout the entire zooming range, and against focusing to suit frominfinity to the minimum object distance, or throughout the entirefocusing range.

In application of the invention to the zoom lens comprising four lensgroups in total, proper rules of design for the lens groups are setforth and the contributions of the lens groups to the variation of thefocal length are made appropriate to obtain high optical performancethroughout the entire zooming range when the maximum angle of fieldincreases to 74° or so and the range increases to 3.5 or so.

A zoom lens of the invention comprises, from front to rear, a first lensgroup of negative refractive power, a second lens group of positiverefractive power and a third lens group of negative refractive power, aseparation between each adjacent two of the lens groups being varied toeffect zooming, wherein letting the heights of incidence of an on-axialray of light on the i-th surface for the wide-angle and telephoto endswith an object at infinity be denoted by Hiw (Hiw>0) and Hit (Hit>0),respectively, and letting the heights of incidence of an off-axialprincipal ray of light from the maximum angle of field on the i-thsurface for the wide-angle and telephoto ends be denoted by Hbiw andHbit, respectively, an aspheric surface is applied to a lens surfacewhich lens surface is concave toward the object side and which lenssurface is negative in the refracting power and which satisfies thefollowing conditions:

    Hiw>|Hbiw|                               (1)

    Hiw>|Hbit|                               (2)

and the aspheric surface is formed to such shape that the negativerefractive power gets progressively stronger from the lens center towardthe lens margin.

Another zoom lens of the invention comprises a first lens group whoseoverall refractive power for the wide angle end is positive and a secondlens group of negative refractive power, a separation between the firstand second lens groups being varied to vary the focal length, whereinthe second lens group has a first lens unit of negative refractive powerand a second lens unit of negative refractive power, a separationbetween the first and second lens units being varied for focusing.

Still another zoom lens of the invention has a first lens group which,in the wide-angle end, takes a negative refractive power, a second lensgroup which takes a positive refractive power, a third lens group whichtakes a positive refractive power and a fourth lens group which takes anegative refractive power, a separation between each adjacent two of thelens groups being varied for zooming, wherein letting the lateralmagnifications for the wide-angle and telephoto ends of the i-th lensgroup (i≧2) be denoted by βiW and βiT, respectively, and putting themagnification ratio Δβi of the i-th lens group to be

    Δβi=|βiT/βiW|

the second lens group has the greatest magnification ratio among thelens groups of positive refractive powers under the condition of Δβi>1and includes a three-cemented lens having a positive overall refractivepower and composed of either two positive lenses or two negative lensesand one lens having a refractive power opposite thereto in cementedcontact form.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram to explain the paraxial retractive powerarrangements of a numerical example 1 of the invention.

FIG. 2 is a diagram to explain the paraxial refractive powerarrangements in a numerical example 2 of the invention.

FIG. 3 is a diagram to explain the paraxial refractive powerarrangements in a numerical example 3 of the invention.

FIG. 4 is a diagram to explain the paraxial refractive powerarrangements in a numerical example 4 of the invention.

FIG. 5 is a diagram to explain the paraxial refractive powerarrangements in a numerical example 5 of the invention.

FIG. 6 is a lens block diagram of the numerical example 1 of theinvention in the wide-angle end.

FIG. 7 is a lens block diagram of the numerical example 2 of theinvention in the wide-angle end.

FIG. 8 is a lens block diagram of the numerical example 3 of theinvention in the wide-angle end.

FIG. 9 is a lens block diagram of the numerical example 4 of theinvention in the wide-angle end.

FIG. 10 is a lens block diagram of the numerical example 5 of theinvention in the wide-angle end.

FIGS. 11A-11D are graphic representations of the aberrations of thenumerical example 1 of the invention in the wide-angle end.

FIGS. 12A-12D are graphic representations of the aberrations of thenumerical example 1 of the invention in a middle position.

FIGS. 13A-13D are graphic representations to the aberrations of thenumerical example 1 of the invention in the telephoto end.

FIGS. 14A-14D are graphic representations of the aberrations of thenumerical example 2 of the invention in the wide-angle end.

FIGS. 15A-15D are graphic representations of the aberrations of thenumerical example 2 of the invention in a middle position.

FIGS. 16A-16D are graphic representations of the aberrations of thenumerical example 2 of the invention in the telephoto end.

FIGS. 17A-17D are graphic representations of the aberrations of thenumerical example 3 of the invention in the wide-angle end.

FIGS. 18A-18D are graphic representations of the aberrations of thenumerical example 3 of the invention in a middle position.

FIGS. 19A-19D are graphic representations of the aberrations of thenumerical example 3 of the invention in the telephoto end.

FIGS. 20A-20D are graphic representations of the aberrations of thenumerical example 4 of the invention in the wide-angle end.

FIGS. 21A-21D are graphic representations of the aberrations of thenumerical example 4 of the invention in a middle position.

FIGS. 22A-22D are graphic representations of the aberrations of thenumerical example 4 of the invention in the telephoto end.

FIGS. 23A-23D are graphic representations of the aberrations of thenumerical example 5 of the invention in the wide-angle end.

FIGS. 24A-24D are graphic representations of the aberrations of thenumerical example 5 of the invention in a middle position.

FIGS. 25A-25D are graphic representations of the aberrations of thenumerical example 5 of the invention in the telephoto end.

FIGS. 26(A) and 26(B) are diagrams of geometry to explain the asphericsurface relevant to the zoom lens of the invention.

FIG. 27 is a lens block diagram of a numerical example 6 of theinvention in the wide-angle end.

FIG. 28 is a lens block diagram of a numerical example 7 of theinvention in the wide-angle end.

FIG. 29 is a lens block diagram of a numerical example 8 of theinvention in the wide-angle end.

FIG. 30 is a lens block diagram of a numerical example 9 of theinvention in the wide-angle end.

FIG. 31 is a lens block diagram of a numerical example 10 of theinvention in the wide-angle end.

FIGS. 32A-32D are graphic representations of the aberrations of thenumerical example 6 of the invention in the wide-angle end with anobject at infinity.

FIGS. 33A-33D are graphic representations of the aberrations of thenumerical example 6 of the invention in a middle position with an objectat infinity.

FIGS. 34A-34D are graphic representations of the aberrations of thenumerical example 6 of the invention in the telephoto end with an objectat infinity.

FIGS. 35A-35D are graphic representations of the aberrations of thenumerical example 6 of the invention in the wide-angle end with anobject at 800 mm from the image plane.

FIGS. 36A-36D are graphic representations of the aberrations of thenumerical example 6 of the invention in the middle position with anobject at 800 mm from the image plane.

FIGS. 37A-37D are graphic representations of the aberrations of thenumerical example 6 of the invention in the telephoto end with an objectat 800 mm from the image plane.

FIGS. 38A-38D are graphic representations of the aberrations of thenumerical example 7 of the invention in the wide-angle end with anobject at infinity.

FIGS. 39A-39D are graphic representations of the aberrations of thenumerical example 7 of the invention in a middle position with an objectat infinity.

FIGS. 40A-40D are graphic representations of the aberrations of thenumerical example 7 of the invention in the telephoto end with an objectat infinity.

FIGS. 41A-41D are graphic representations of the aberrations of thenumerical example 7 of the invention in the wide-angle end with anobject at 800 mm from the image plane.

FIGS. 42A-42D are graphic representations of the aberrations of thenumerical example 7 of the invention in the middle position with anobject at 800 mm from the image plane.

FIGS. 43A-43D are graphic representations of the aberrations of thenumerical example 7 of the invention in the telephoto end with an objectat 800 mm from the image plane.

FIGS. 44A-44D are graphic representations of the aberrations of thenumerical example 7 of the invention in the wide-angle end when focusedto 800 mm from the image plane by moving the lens unit L21 and the lensunit L22 to respective distances of 1:0.5.

FIGS. 45A-45D are graphic representations of the aberrations of thenumerical example 7 of the invention in the middle position when focusedto 800 mm from the image plane by moving the lens unit L21 and the lensunit L22 to respective distances of 1:0.5.

FIGS. 46A-46D are graphic representations of the aberrations of thenumerical example 7 of the invention in the telephoto end when focusedto 800 mm from the image plane by moving the lens unit L21 and the lensunit L22 to respective distances of 1:0.5.

FIGS. 47A-47D are graphic representations of the aberrations of thenumerical example 8 of the invention in the wide-angle end with anobject at infinity.

FIGS. 48A-48D are graphic representations of the aberrations of thenumerical example 8 of the invention in a middle position with an objectat infinity.

FIGS. 49A-49D are graphic representations of the aberrations of thenumerical example 8 of the invention in the telephoto end with an objectat infinity.

FIGS. 50A-50D are graphic representations of the aberrations of thenumerical example 8 of the invention in the wide-angle end with anobject at 800 mm from the image plane.

FIGS. 51A-51D are graphic representations of the aberrations of thenumerical example 8 of the invention in the middle position with anobject at 800 mm from the image plane.

FIGS. 52A-52D are graphic representations of the aberrations of thenumerical example 8 of the invention in the telephoto end with an objectat 800 mm from the image plane.

FIGS. 53A-53D are graphic representations of the aberrations of thenumerical example 9 of the invention in he wide-angle end with an objectat infinity.

FIGS. 54A-54D are graphic representations of the aberrations of thenumerical example 9 of the invention in a middle position with an objectat infinity.

FIGS. 55A-55D are graphic representations of the aberrations of thenumerical example 9 of the invention in the telephoto end with an objectat infinity.

FIGS. 56A-56D are graphic representations of the aberrations of thenumerical example 9 of the invention in the wide-angle end with anobject at 800 mm from the image plane.

FIGS. 57A-57D are graphic representations of the aberrations of thenumerical example 9 of the invention in the middle position with anobject at 800 mm from the image plane.

FIGS. 58A-58D are graphic representations of the aberrations of thenumerical example 9 of the invention in the telephoto end with an objectat 800 mm from the image plane.

FIGS. 59A-59D are graphic representations of the aberrations of thenumerical example 10 of the invention in the wide-angle end with anobject at infinity.

FIGS. 60A-60D are graphic representations of the aberrations of thenumerical example 10 of the invention in a middle position with anobject at infinity.

FIGS. 61A-61D are graphic representations of the aberrations of thenumerical example 10 of the invention in the telephoto end with anobject at infinity.

FIGS. 62A-62D are graphic representations of the aberrations of thenumerical example 10 of the invention in the wide-angle end with anobject at 800 mm from the image plane.

FIGS. 63A-63D are graphic representations of the aberrations of thenumerical example 10 of the invention in the middle position with anobject at 800 mm from the image plane.

FIGS. 64A-64D are graphic representations of the aberrations of thenumerical example 10 of the invention in the telephoto end with anobject at 800 mm from the image plane.

FIGS. 65(A) and 65(B) are diagrams to explain the paraxial refractivepower arrangements of a zoom lens of the invention.

FIG. 66 is a diagram to explain the variation of the paraxial refractivepower arrangement for numerical examples 11 to 13 of the invention.

FIG. 67 is a diagram to explain the variation of the paraxial refractivepower arrangement for numerical examples 14 and 15 of the invention.

FIG. 68 is a diagram to explain the variation of the paraxial refractivepower arrangement for numerical examples 16 and 17 of the invention.

FIG. 69 is a lens block diagram of the numerical example 11 of theinvention in the wide-angle end.

FIG. 70 is a lens block diagram of the numerical example 12 of theinvention in the wide-angle end.

FIG. 71 is a lens block diagram of the numerical example 13 of theinvention in the wide-angle end.

FIG. 72 is a lens block diagram of the numerical example 14 of theinvention in the wide-angle end.

FIG. 73 is a lens block diagram of the numerical example 15 of theinvention in the wide-angle end.

FIG. 74 is a lens block diagram of the numerical example 16 of theinvention in the wide-angle end.

FIG. 75 is a lens block diagram of the numerical example 17 of theinvention in the wide-angle end.

FIGS. 76A-76D are graphic representations of the aberrations of thenumerical example 11 of the invention in the wide-angle end.

FIGS. 77A-77D are graphic representations of the aberrations of thenumerical example 11 of the invention in a middle position.

FIGS. 78A-78D are graphic representations of the aberrations of thenumerical example 11 of the invention in the telephoto end.

FIGS. 79A-79D are graphic representations of the aberrations of thenumerical example 12 of the invention in the wide-angle end.

FIGS. 80A-80D are graphic representations of the aberrations of thenumerical example 12 of the invention in a middle position.

FIGS. 81A-81D are graphic representations of the aberrations of thenumerical example 12 of the invention in the telephoto end.

FIGS. 82A-82D are graphic representations of the aberrations of thenumerical example 13 of the invention in the wide-angle end.

FIGS. 83A-83D are graphic representations of the aberrations of thenumerical example 13 of the invention in a middle position.

FIGS. 84A-84D are graphic representations of the aberrations of thenumerical example 13 of the invention in the telephoto end.

FIGS. 85A-85D are graphic representations of the aberrations of thenumerical example 14 of the invention in the wide-angle end.

FIGS. 86A-86D are graphic representations of the aberrations of thenumerical example 14 of the invention in a middle position.

FIGS. 87A-87D are graphic representations of the aberrations of thenumerical example 14 of the invention in the telephoto end.

FIGS. 88A-88D are graphic representations of the aberrations of thenumerical example 15 of the invention in the wide-angle end.

FIGS. 89A-89D are graphic representations of the aberrations of thenumerical example 15 of the invention in a middle position.

FIGS. 90A-90D are graphic representations of the aberrations of thenumerical example 15 of the invention in the telephoto end.

FIGS. 91A-91D are graphic representations of the aberrations of thenumerical example 16 of the invention in the wide-angle end.

FIGS. 92A-92D are graphic representations of the aberrations of thenumerical example 16 of the invention in a middle position.

FIGS. 93A-93D are graphic representations of the aberrations of thenumerical example 16 of the invention in the telephoto end.

FIGS. 94A-94D are graphic representations of the aberrations of thenumerical example 17 of the invention in the wide-angle end.

FIGS. 95A-95D are graphic representations of the aberrations of thenumerical example 17 of the invention in a middle position.

FIGS. 96A-96D are graphic representations of the aberrations of thenumerical example 17 of the invention in the telephoto end.

FIG. 97 is a graph to explain the variation of the 3rd orderlongitudinal chromatic aberration coefficient of the numerical example15 of the invention.

FIG. 98 is a graph to explain the variation of the 3rd order lateralchromatic aberration coefficient of the numerical example 15 of theinvention.

FIG. 99 is a graph to explain the variation of the 3rd order sphericalaberration coefficient of the numerical example 15 of the invention.

In these drawings, L1 denotes the first lens group, L2 the second lensgroup, L3 the third lens group, and L4 the fourth lens group. SP standsfor the stop and SPP for the fixed stop. IP denotes the image plane. drepresents the spectral d-line, g the spectral g-line, S.C the sinecondition, ΔS the sagittal image surface, and ΔM the meridional imagesurface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 to FIG. 5 are schematic diagrams showing the variations withzooming of the paraxial refractive power arrangements for the numericalexamples 1 to 5 of zoom lenses of the invention. FIG. 6 to FIG. 10 arelongitudinal section views of the numerical examples 1 to 5 of the zoomlenses of the invention in the wide-angle end. FIG. 11A to FIG. 13D showthe aberrations of the zoom lens of the numerical example 1 of theinvention in the wide-angle end, a middle position and the telephotoend, respectively. FIG. 14A to FIG. 16D show the aberrations of thenumerical example 2 of the invention in the wide-angle end, a middleposition and the telephoto end, respectively. FIG. 17A to FIG. 19D showthe aberrations of the numerical example 3 of the invention in thewide-angle end, a middle position and the telephoto end, respectively.FIG. 20A to FIG. 22D show the aberrations of the numerical example 4 ofthe invention in the wide-angle end, a middle position and the telephotoend, respectively. FIG. 23A to FIG. 25D show the aberrations of thenumerical example 5 of the invention in the wide-angle end, a middleposition and the telephoto end, respectively.

In these drawings, L1 is the first lens group of negative refractivepower, L2 is the second lens group of positive refractive power, L3 isthe third lens group of negative refractive power, SP is the aperturestop, and IP is the image plane.

A zoom lens of the invention comprises a plurality of lens groups. Inthe numerical examples 1 to 5 of FIG. 1 to FIG. 5, however, for thepurpose of convenience, the total number of lens groups is limited to 3,so that the entire system comprises a first lens group L1 of negativerefractive power, a second lens group L2 of positive refractive powerand a third lens group L3 of negative refractive power. Then, afundamental rule is set forth that all these lens groups, when zoomingfrom the wide-angle end to the telephoto end, axially move in suchrelation that the separations between the first lens group L1 and thesecond lens group L2 and between the second lens group L2 and the thirdlens group L3 vary as indicated by the arrows.

In addition, as the zoom lens is constructed with such a plurality oflens groups, aspheric surface of described shape is applied to a lenssurface which satisfies the conditions (1) and (2) described before,thereby giving an advantage that despite the widening of the maximumangle of field and the increasing of the zooming range, the variation ofaberrations is well corrected to obtain high optical performancethroughout the entire zooming range.

Next, the features of the design for the zoom lens which arecharacteristic of the invention are described below.

A zoom lens of the invention comprises, from front to rear, as in thewide-angle end, a first lens group L1 whose overall refractive power isnegative, followed, after a considerable space, by a second lens groupL2 whose overall refractive power is positive, which in turn isfollowed, after a considerable space, by a third lens group L3 whoseoverall refractive power is negative. By taking such a symmetric, orminus-plus-minus, refractive power arrangement in the wide-angle end, itis made possible to strengthen the refractive power for the second lensgroup L2. With the help of this, the widening of the maximum angle offield and the minimizing of the bulk and size are achieved in such amanner as to correct well all aberrations.

Another feature is that the first lens group L1 of negative overallrefractive power and the second lens group L2 of positive overallrefractive power are considerably spaced apart from each other, thustaking the form of the retrofocus type. This makes it easy to secure theback focal distance, which would otherwise become a problem whenwidening the maximum angle of field. The second lens group L2 is thenconstructed as comprising, from front to rear, a front lens unit L2a ofpositive refractive power, a middle lens unit L2b of negative refractivepower and a rear lens unit L2c of positive refractive power, thearrangement being such that when zooming to the wide-angle end, theyapproach each other. When in the wide-angle end, they take a symmetricform, thus effectively correcting the aberrations that the second lensgroup L2 produces.

To fulfill the requirements of minimizing the bulk and size of the lenssystem and of increasing the range at once, it is necessary in thegeneral case to strengthen the refractive power of each lens group and,at the same time, to remove as many constituent lenses from each lensgroup as possible. If this approach to the improvement of the compactform and the increase of the range is pushed, the difficulty ofcorrecting the positive Lens groups for spherical aberration of thenegative direction increases rapidly. In the invention, therefore, forthe purpose of correcting such spherical aberration, aspheric surface isintroduced into a lens surface on which the height of incidence of theoff-axial light ray is smaller in the absolute value than the height ofincidence of the on-axial light ray in the wide-angle end and thetelephoto end and is concave toward the object side and negative in therefracting power.

In more detail, the lens surface which satisfies the conditions (1) and(2) simultaneously is selected to introduce the aspheric surface of sucha shape that the negative refractive power gets progressively strongeras the distance from the optical axis increases. Mainly, the sphericalaberration of the negative direction is thus corrected toward thepositive direction.

Besides this, there is an advantage that, without giving too muchinfluence to the variation of astigmatism with zooming, good correctionof spherical aberration and coma is assured throughout the entirezooming range, so long as the provision of the aspheric sphere ofprescribed shape is made in the lens surfaces whose parameters fallwithin the ranges that satisfy the conditions (1) and (2). When theseconditions are violated, good correction of on-axial aberrations becomesdifficult to do so effectively as not to greatly affect off-axialaberrations over the entire zooming range. It is also to be noted thatthe selection of the front surface of the negative lens which is concavetoward the object side is desirable for introduction of the asphericsurface of the aforesaid shape, because, if so, it has much strong aneffect of correcting spherical aberration.

FIGS. 26(A) and 26(B) are diagrams of geometry to explain the situationswhen the marginal ray of the axial beam and the principal ray of theoff-axial beam enter a lens surface that satisfies the conditions (1)and (2).

Another feature is that when applying the aspheric surface to the lenssurface which satisfies the conditions (1) and (2), it is preferable tofirst choose the second lens group L2 whose overall refractive power ispositive, then its middle lens unit L2b and then a negative lenstherein, because the variation of spherical aberration can beeffectively corrected without giving too much influence to the variationof off-axial aberrations.

It will be appreciated from the foregoing that in the invention, atleast one aspheric surface is introduced into the lens surface whichsatisfies the conditions (1) and (2) described before and is concavetoward the object side and has a negative refracting function. Hence,the on-axial aberrations and the off-axial aberrations are corrected ingood balance, while the increase of the range and the widening of theangular field are simultaneously achieved.

In the invention, focusing of the zoom lens is performed by using adesired one of those lens groups or lens units which do not take unityof lateral magnification during zooming. If the first lens group L1includes a lens unit whose refractive power is considerably strong, itis recommended to use this lens unit for focusing. In this case, for anyobject distance, the focusing movement can be kept constant throughoutthe entire zooming range. So, a simplification of the structure of theoperating mechanism can be expected.

If the back focal distance is sufficiently long in the wide-angle end,the last lens group or unit has a negative refractive power and thisrefractive power is considerably strong, then this last lens group orunit may be moved toward the image side. In this case, reduction of theouter diameter of the first lens group can be expected. Of the first tothe last lens groups, two or more lens groups or units may be movedsimultaneously. If the focusing lens group or unit contains the stop, itis preferred to keep the stop axially stationary during focusing,because the driving torque can be reduced by the amount necessary tomove the diaphragm mechanism.

The features of each of the zoom lenses shown in FIG. 1 to FIG. 5 aredescribed below.

FIG. 1 shows the paraxial refractive power arrangements of the numericalexample 1 of the invention. In FIG. 1, the first lens group L1 isconstructed with one lens unit (L1) having a negative refractive power.The second lens group L2 is constructed with three lens units, of whichthe front lens unit (L2a), as viewed to the object side, is of positiverefractive power, the middle lens unit (L2b) is of negative refractivepower and the rear lens unit (L2c) is of positive refractive power. Thethird lens group L3 is constructed with one lens unit (L3) having anegative refractive power.

The separation between the first lens group L1 and the front lens unitL2a is made smaller when in the telephoto end than when in thewide-angle end. As will be understood from an equation (9) to bedescribed later, the first lens group L1 and the front lens unit L2a incombination with each other thus gets a weaker overall refractive power(longer overall focal length). As a result, the front lens unit L2a andthe third lens group L3 function to increase the magnification. By this,the zooming range is extended toward the telephoto side with highefficiency, giving an advantage of increasing the range. Again, when inthe telephoto end, the separation between the middle lens unit L2b andthe rear lens unit L2c is made longer, so that the off-axial light raypasses through the rear lens unit L2c at a farther position away fromthe optical axis. Thus, on-axial aberrations and off-axial aberrationsare corrected in good balance.

FIG. 2 shows the paraxial refractive power arrangements of the numericalexample 2 of the invention. In FIG. 2, the first lens group isconstructed with two lens units, of which the front lens unit (L1a), asviewed to the object side, is of positive refractive power and the rearlens unit (L1b) is of negative refractive power. The second lens groupL2 is constructed with three lens units, of which the front lens unit(L2a), as viewed to the object side, is of positive refractive power,the middle lens unit (L2b) is of negative refractive power and the rearlens unit (L2c) is of positive refractive power. The third lens group L3is constructed with one lens unit (L3) of negative refractive power.

For the wide-angle end, the first lens group L1 and the second lensgroup L2 are arranged in considerably spaced relation. As a result, thefirst lens group L1 and the second lens group L2 take the form of theretrofocus type. By this, it is made easier to secure the back focaldistance, which becomes a problem when to widen the angle of field.Another feature is that the separation between the rear lens unit L1band the front lens unit L2a and the separation between the rear lensunit L2c and the third lens group L3 are made shorter when in thetelephoto end than when in the wide-angle end. As is understandable fromthe equation (9), the lens units L1b and L2a or the unit L2c and thegroup L3 in combination with each other thus gets a weaker overallrefractive power (longer overall focal length). As a result, the middlelens unit L2b and the third lens group L3 produce an effect ofincreasing the magnification. This increases the efficiency with whichthe zooming range is extended toward the telephoto side, thereby makingit easier to increase the range. When in the wide-angle end, theseparation between the front lens unit L1a and the rear lens unit L1b ismade shorter to correct well particularly the negative distortion thatis produced from the rear lens unit L1b when the maximum angle of fieldwidens.

FIG. 3 shows the paraxial refractive power arrangements of the numericalexample 3 of the invention. In FIG. 3, the first lens group L1 isconstructed with two lens units, of which the front lens unit (L1a), asviewed to the object side, is of positive refractive power and the rearlens unit (L1b) is of negative refractive power. The second lens groupL2 is constructed with three lens units, of which the front lens unit(L2a), as viewed to the object side, is of positive refractive power,the middle lens unit (L2b) is of negative refractive power, and the rearlens unit (L2c) is of positive refractive power. The third lens group L3is constructed with one lens unit (L3) having a negative refractivepower. Notice that the middle lens unit L2b and the rear lens unit L2cmove in unison during zooming, so they are depicted in FIG. 3 as onelens unit L2bc of positive overall refractive power.

In here, the separation between the front lens unit L1a and the rearlens unit L1b is made longer and the separation between the rear lensunit L1b and the front lens unit L2a is made shorter when in thetelephoto end than when in the wide-angle end. With this, when in thewide-angle end, negative distortion is corrected. When in the telephotoend, as they form the telephoto type, the total length of the completelens is shortened.

FIG. 4 shows the paraxial refractive power arrangements of the numericalexample 4 of the invention. In FIG. 4, the first lens group L1 isconstructed with one lens unit (L1) having a negative refractive power.The second lens group L2 is constructed with three lens units, of whichthe front lens unit (L2a), as viewed to the object side, is of positiverefractive power, the middle lens unit (L2b) is of negative refractivepower and the rear lens unit (L2C) is of positive refractive power. Thethird lens unit L3 is constructed with one lens unit (L3) having anegative refractive power. Notice that the middle lens unit L2b and therear lens unit L2c move in unison during zooming, so that they aredepicted in FIG. 4 as one lens unit L2bc whose overall refractive poweris positive.

The separation between the first lens group L1 and the front lens unitL2a and the separation between the combined lens unit L2bc and the thirdlens group L3 are made shorter when in the telephoto end than when inthe wide-angle end. With this, the lens system as a whole keeps thesymmetric form during zooming, thereby making it easier to obtain goodoptical performance. Moreover, when in the wide-angle end, the entirelens system gets a shorter total length.

FIG. 5 shows the paraxial refractive power arrangements of the numericalexample 5 of the invention. In FIG. 5, the first lens group L1 isconstructed with one lens unit (L1) having a negative refractive power.The second lens group L2 is constructed with three lens units, of whichthe front lens unit (L2a), as viewed to the object side, is of positiverefractive power, the middle lens unit (L2b) is of negative refractivepower and the rear lens unit (L2c) is of positive refractive power. Thethird lens group L3 is constructed with one lens unit (L3) having anegative refractive power. Notice that the front lens unit L2a, themiddle lens unit L2b and the rear lens unit L2c move in unison duringzooming, so that they are depicted in FIG. 5 as one lens unit L2abcwhose overall refractive power is positive.

When zooming from the wide-angle end to the telephoto end, all the lensgroups axially move toward the object side in such relation that theseparation between the first lens group L1 and the second lens group L2and the separation between the second lens group L2 and the third lensgroup L3 decrease. So, during zooming, the lens system as a whole keepsthe symmetric form, thereby making it easier to obtain good opticalperformance. Along with this, variation of the focal length is performedeffectively.

The features or conditions described above suffice for accomplishing theobjects of the invention. To achieve further improvements of the compactform and optical performance of such a wide-angle high range zoom lens,the following additional features or conditions are then recommended.

(1-1) Letting the focal length of the third lens group be denoted by f3and the shortest focal length of the entire system by Fw, the followingcondition is satisfied:

    0.7≦|Fw/f3|≦2.5            (3)

The equalities or inequalities (3) are concerned with the negativerefractive power of the third lens group L3. When the upper limit isexceeded, as this means that the third lens group L3 has too strong arefractive power, the back focal distance becomes too short in thewide-angle end. Therefore, the third lens group L3 must have an undulylarge diameter. Otherwise, the corner illumination could not be securedat a satisfactory level. When the refractive power of the third lensgroup L3 is too weak as exceeding the lower limit, the zooming movementof the third lens group L3 contributes to a weaker effect of varying thefocal length. As a result, to secure the predetermined value of the zoomratio, the zooming movements of the other lens groups must be increased.This increases the total length of the entire lens system objectionably.

(1-2) The stop is better positioned within the second lens group. It isalso good that when zooming, the stop is made to axially move either inunison with, or in differential relation to, the other lens groups.Incidentally, besides the interior of the second lens group, the stopmay otherwise be positioned in the air space between the first lensgroup L1 and the second lens group L2. This latter case is desirablefrom the point of view of minimizing the lens diameter and improving thebalance of aberration correction.

(1-3) The invention is not confined to the scope of the conditions (1)and (2) described before, when the aspheric surface is introduced. Tofurther improve the aberration correction, an additional asphericsurface is better introduced into one of the other lens surfaces. Forexample, at least one negative lens in the third lens group L3 may beselected to introduce at least one aspheric surface. If so, it becomespossible to better correct the variation of mainly off-axialaberrations. Another additional aspheric surface may be introduced intothe second lens group L2 except at that lens surface which was selectedfor the aspheric surface of principal interest described before. If so,off-axial aberrations can be corrected well. For example, that lenssurface in the lens unit L2c which is convex toward the image side isbetter selected to introduce thereto an aspheric sphere of such shapethat the positive refractive power gets progressively weaker as theheight from the optical axis increases. If an aspheric surface isintroduced into the first lens group L1, mainly distortion becomes easyto correct.

(1-4) Letting the focal lengths for the wide-angle end of the first andsecond lens groups be denoted by f1 and f2, respectively, the followingconditions are satisfied:

    0.2≦|Fw/f1|≦1.0            (4)

    1.0≦Fw/f2≦2.5                                (5)

When the upper limit of the condition (4) is exceeded, as this meansthat the first lens group L1 has too strong a negative refractive powerin the wide-angle end, the retrofocus system takes a strong action. So,the total length of the complete lens increases objectionably. Moreover,the first lens group L1 produces strong spherical aberration as isover-corrected, which is difficult to correct by any design of the otherlens groups. When the lower limit is exceeded, it becomes difficult tosecure the predetermined value of the back focal distance.

The equalities or inequalities (5) are concerned with the positiverefractive power of the second lens group L2. When the upper limit ofthe condition (5) is exceeded, as this means that the second lens groupL2 has too strong a refractive power, the second lens group L2 and thethird lens group L3 form the telephoto type that functions too strongly.Therefore, the predetermined value of the back focal distance is hardlysecured. Conversely, when the lower limit of the condition (5) isexceeded, as this means that the second lens group L2 has too weak arefractive power, the refractive powers of the negative lens groups mustbe weakened to obtain the predetermined value of the shortest focallength. As a result, the total length of the complete lens increasesobjectionably.

(1-5) Letting the image magnification for the wide-angle end of thethird lens group be denoted by β3w, the following condition issatisfied:

    0.1≦f3·(1-β3w)/Fw≦0.5          (6)

The equalities or inequalities (6) are concerned with an appropriatecondition to determine the back focal distance for the wide-angle end.When the upper limit of the condition (6) is exceeded, the back focaldistance becomes longer than necessary in the wide-angle end. Therefore,the size of the entire lens system is difficult to minimize. When thelower limit is exceeded, it becomes difficult to secure thepredetermined value of the back focal distance in the wide-angle end. Asa result, the third lens group L3 gets an objectionably larger diameter.

(1-6) Letting the lateral magnification for the wide-angle end of thethird lens group be denoted by β3w, the following condition issatisfied:

    1.1<β3w<1.8                                           (7)

The inequalities (7) are concerned with the lateral magnification of thethird lens group L3. When the third lens group L3 has too large alateral magnification as exceeding the upper limit, a long back focaldistance is obtained, but the preceding lens groups get too strongrefractive powers. Therefore, aberrations become difficult to correct.When the third lens group L3 has too weak a lateral magnification asexceeding the lower limit, it becomes difficult to minimize the size ofthe entire lens system.

(1-7) Letting the lateral magnification for the wide angle end of thesecond lens group be denoted by β2w (β2w<0), the following condition issatisfied:

    0.1<|β2w|<0.6                       (8)

It is considered that the two adjacent lens groups (called the "k-th"lens group and the "j-th" lens group) are combined with each other.Then, the combined lens group has an overall refractive power φkj givenby the following equation:

    φkj=φk+φj-φk·φj·e    (9)

where

φk: the refractive power of the k-th lens group;

φj: the refractive power of the j-th lens group; and

e: the interval between the principal points of the k-th lens group andthe j-th lens group.

The second lens group L2 and the third lens group L3 have refractivepowers of opposite sign to each other. As is understandable from theequation (9), therefore, if the air separation is made shorter when inthe telephoto end than when in the wide-angle end, the combined lensgroup can take a weaker overall refractive power.

The inequalities (8) give a range for the lateral magnification for thewide-angle end of the second lens group L2, as is determined byreferring to the equation (9). When the upper limit of the condition (8)is exceeded, the necessary back focal distance is hard to take in thewide-angle end. As a result, the third lens group L3 gets a largerdiameter. When the lower limit is exceeded, the refractive powers of theother lens groups becomes much too strong in order to obtain thepredetermined focal lengths. Therefore, many aberrations increaseobjectionably.

Next, the numerical data for the numerical examples 1 to 5 of theinvention are given in the following tables, wherein Ri is the radius ofcurvature of the i-th lens surface, when counted from the object side,Di is the i-th lens thickness or air separation, when counted from theobject side, and Ni and νi are respectively the refractive index andAbbe number of the glass of the i-th lens element, when counted from theobject side.

The values of the factors in the before-described conditions for thenumerical examples 1 to 5 are listed in Table-1.

The shape of the aspheric surface is expressed in the coordinates withan X axis in the axial direction and an H axis in the perpendiculardirection to the optical axis, the direction in which light advancesbeing taken as positive by the following equation: ##EQU1## where R isthe radius of the osculating sphere, and K, A, B, C, D and E are theaspheric coefficients.

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.84 - 101.45  Fno. = 3.5 - 9  2ω = 73.8° - 24.1°    ______________________________________                                        R1 = -129.95                                                                             D1 = 1.30   N1 = 1.48749                                                                              ν1 = 70.2                               R2 = 29.51 D2 = 1.99                                                          R3 = 29.12 D3 = 2.20   N2 = 1.84666                                                                              ν2 = 23.8                               R4 = 39.39 D4 = Variable                                                      R5 = 14.00 D5 = 1.10   N3 = 1.84666                                                                              ν3 = 23.8                               R6 = 11.92 D6 = 3.00   N4 = 1.48749                                                                              ν4 = 70.2                               R7 = -234.68                                                                             D7 = Variable                                                      R8 = (Stop)                                                                              D8 = 2.50                                                          R9 = -19.70                                                                              D9 = 1.50   N5 = 1.69320                                                                              ν5 = 33.7                               R10 = -21.42                                                                             D10 = 2.00  N6 = 1.84666                                                                              ν6 = 23.8                               R11 = -62.22                                                                             D11 = Variable                                                     R12 = 25202.75                                                                           D12 = 4.10  N7 = 1.77250                                                                              ν7 = 49.6                               R13 = -15.62                                                                             D13 = Variable                                                     R14 = -18.26                                                                             D14 = 1.50  N8 = 1.69680                                                                              ν8 = 55.5                               R15 = 73.66                                                                              D15 = 2.30  N9 = 1.84666                                                                              ν9 = 23.8                               R16 = 218.48                                                                  ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R9:  K = 5.081   A = 0  B = 2.434 × 10.sup.-6                                                              C = 6.462 × 10.sup.-7                     D = 0       E = 0                                                        R13: K = -2.521  A = 0  B = -5.677 × 10.sup.-5                                                             C = 1.319 × 10.sup.-7                     D = 0       E = 0                                                        R14: K = 4.719 × 10.sup.-1                                                               A = 0  B = 2.780 × 10.sup.-5                                                              C = 8.034 × 10.sup.-8                     D = 0       E = 0                                                        ______________________________________                                        Variable    Focal Length                                                      Separations 28.84        53.10  101.45                                        ______________________________________                                        D4          9.00         14.18  0.80                                          D7          1.00         6.08   8.04                                          D11         2.27         4.78   9.15                                          D13         14.06        4.20   0.95                                          ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.83 - 102.38  Fno. = 3.71 - 10.00  2ω = 73.8° -            23.9°                                                                  ______________________________________                                        R1 = 519.61                                                                              D1 = 1.60   N1 = 1.80518                                                                              ν1 = 25.4                               R2 = -351.51                                                                             D2 = Variable                                                      R3 = -112.82                                                                             D3 = 1.30   N2 = 1.49699                                                                              ν2 = 81.6                               R4 = 28.78 D4 = Variable                                                      R5 = 13.78 D5 = 2.50   N3 = 1.49699                                                                              ν3 = 81.6                               R6 = 2684.81                                                                             D6 = Variable                                                      R7 = (Stop)                                                                              D7 = 1.00                                                          R8 = -35.81                                                                              D8 = 2.80   N4 = 1.84665                                                                              ν4 = 23.8                               R9 = 1454.82                                                                             D9 = Variable                                                      R10 = -317.50                                                                            D10 = 4.50  N5 = 1.77249                                                                              ν5 = 49.6                               R11 = -16.39                                                                             D11 = Variable                                                     R12 = -15.81                                                                             D12 = 1.80  N6 = 1.63999                                                                              ν6 = 60.1                               R13 = -425.93                                                                 ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R8:  K = -2.282 × 10.sup.+1                                                                A = 0        B =-1.474 × 10.sup.-4                        C = 1.145 × 10.sup.-8                                                                 D = -5.666 × 10.sup.-9                                                               E = 0                                         R11: K = -2.726 × 10.sup.-2                                                                A = 0        B = 6.307 × 10.sup.-6                        C = 3.236 × 10.sup.-8                                                                 D = -5.515 × 10.sup.-11                                                              E = 0                                         R12: K = -1.579 × 10.sup.-1                                                                A = 0        B = 1.052 × 10.sup.-5                        C = 3.604 × 10.sup.-8                                                                 D = -9.831 × 10.sup.-11                                                              E = 0                                         ______________________________________                                        Variable    Focal Length                                                      Separations 28.83        54.33  102.38                                        ______________________________________                                        D2          1.49         1.64   3.36                                          D4          8.73         14.26  0.86                                          D6          3.06         7.24   8.88                                          D9          2.85         5.91   10.58                                         D11         17.12        5.44   4.31                                          ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.85 - 101.00  Fno. = 3.30 - 9.00  2ω = 73.7                       ______________________________________                                        - 24.2°                                                                R1 = 424.11                                                                             D1 = 2.40    N1 = 1.51633                                                                              ν1 = 64.2                               R2 = -60.06                                                                             D2 = Variable                                                       R3 = -38.54                                                                             D3 = 1.20    N2 = 1.80400                                                                              ν2 = 46.6                               R4 = 19.56                                                                              D4 = 1.35                                                           R5 = 21.49                                                                              D5 = 2.90    N3 = 1.84665                                                                              ν3 = 23.8                               R6 = 176.01                                                                             D6 = Variable                                                       R7 = 15.65                                                                              D7 = 0.90    N4 = 1.84665                                                                              ν4 = 23.8                               R8 = 11.27                                                                              D8 = 4.50    N5 = 1.48749                                                                              ν5 = 70.2                               R9 = -21.44                                                                             D9 = 0.90    N6 = 1.84665                                                                              ν6 = 23.8                               R10 = -29.88                                                                            D10 = Variable                                                      R11 = (Stop)                                                                            D11 = 3.00                                                          R12 = -24.67                                                                            D12 = 2.55   N7 = 1.80518                                                                              ν7 = 25.4                               R13 = -47.29                                                                            D13 = 0.50                                                          R14 = - 36.54                                                                           D14 = 1.00   N8 = 1.65159                                                                              ν8 = 58.5                               R15 = 155.75                                                                            D15 = 5.80   N9 = 1.77249                                                                              ν9 = 49.6                               R16 = -14.23                                                                            D16 = Variable                                                      R17 = -28.76                                                                            D17 = 2.30   N10 = 1.84665                                                                             ν10 = 23.8                              R18 = -20.20                                                                            D18 = 0.30                                                          R19 = -25.76                                                                            D19 = 1.30   N11 = 1.69679                                                                             ν11 = 55.5                              R20 = -80.69                                                                            D20 = 3.51                                                          R21 = -18.83                                                                            D21 = 1.50   N12 = 1.77249                                                                             ν12 = 49.6                              R22 = 431.90                                                                  ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R12: K = 4.963     A = 0        B = -6.074 × 10.sup.-5                       C = -3.607 × 10.sup.-7                                                                D = 3.331 × 10.sup.-9                                                                E = 0                                         R16: K = -2.664    A = 0        B = -1.127 × 10.sup.-4                       C = -1.634 × 10.sup.-7                                                                D = -1.376 × 10.sup.-9                                                               E = 0                                         ______________________________________                                        Variable    Focal Length                                                      Separations 28.85        53.98  101.00                                        ______________________________________                                        D2          1.23         2.21   5.15                                          D6          8.73         4.75   0.81                                          D10         0.79         4.84   8.26                                          D16         10.75        4.72   0.85                                          ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.86 - 101.58  Fno. = 3.06 - 9.00  2ω = 73.7° -             24.1°                                                                  ______________________________________                                        R1 = 101.89                                                                             D1 = 2.85    N1 = 1.51633                                                                              ν1 = 64.2                               R2 = -61.28                                                                             D2 = 0.84                                                           R3 = -39.42                                                                             D3 = 1.20    N2 = 1.80400                                                                              ν2 = 46.6                               R4 = 17.90                                                                              D4 = 1.07                                                           R5 = 19.60                                                                              D5 = 3.35    N3 = 1.84665                                                                              ν3 = 23.8                               R6 = 95.82                                                                              D6 = Variable                                                       R7 = 16.21                                                                              D7 = 0.90    N4 = 1.84665                                                                              ν4 = 23.8                               R8 = 11.92                                                                              D8 = 4.80    N5 = 1.48749                                                                              ν5 = 70.2                               R9 = -19.85                                                                             D9 = 0.90    N6 = 1.84665                                                                              ν6 = 23.8                               R10 = -27.78                                                                            D10 = Variable                                                      R11 = (Stop)                                                                            D11 = 3.50                                                          R12 = -26.05                                                                            D12 = 2.42   N7 = 1.80518                                                                              ν7 = 25.4                               R13 = -45.55                                                                            D13 = 0.56                                                          R14 = -34.59                                                                            D14 = 1.00   N8 = 1.65159                                                                              ν8 = 58.5                               R15 = 310.25                                                                            D15 = 5.80   N9 = 1.77249                                                                              ν9 = 49.6                               R16 = -13.79                                                                            D16 = Variable                                                      R17 = -28.83                                                                            D17 = 2.30   N10 = 1.84665                                                                             ν10 = 23.8                              R18 = -20.42                                                                            D18 = 0.24                                                          R19 = -25.56                                                                            D19 = 1.30   N11 = 1.696.79                                                                            ν11 = 55.5                              R20 = -131.56                                                                           D20 = 3.53                                                          R21 = -20.55                                                                            D21 = 1.50   N12 = 1.77249                                                                             ν12 = 4.9.6                             R22 = 204.29                                                                  ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R12: K = 6.017     A = 0        B = -6.890 × 10.sup.-5                       C = -6.114 × 10.sup.-7                                                                D = -4.934 × 10.sup.-9                                                               E = 0                                         R16: K = -2.445    A = 0        B = -1.158 × 10.sup.-4                       C = 1.246 × 10.sup.-7                                                                 D = -1.894 × 10.sup.-9                                                               E = 0                                         ______________________________________                                        Variable    Focal Length                                                      Separations 28.86        54.15  101.58                                        ______________________________________                                        D6          9.49         5.78   0.79                                          D10         1.56         5.74   8.67                                          D16         9.64         3.97   0.79                                          ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 29.47 - 80.02  Fno. = 3.80 - 8.79  2ω = 72.6° - 30.3.degr    ee.                                                                           ______________________________________                                        R1 = 104.99                                                                             D1 = 2.85    N1 = 1.51633                                                                              ν1 = 64.2                               R2 = -79.20                                                                             D2 = 0.84                                                           R3 = -58.17                                                                             D3 = 1.20    N2 = 1.88299                                                                              ν2 = 40.8                               R4 = 18.99                                                                              D4 = 1.07                                                           R5 = 19.98                                                                              D5 = 3.35    N3 = 1.80518                                                                              ν3 = 25.4                               R6 = 81.43                                                                              D6 = Variable                                                       R7 = 14.33                                                                              D7 = 0.90    N4 = 1.84665                                                                              ν4 = 23.8                               R8 = 15.71                                                                              D8 = 4.80    N5 = 1.48749                                                                              ν5 = 70.2                               R9 = -16.46                                                                             D9 = 0.90    N6 = 1.84665                                                                              ν6 = 23.8                               R10 = -25.87                                                                            D10 = 2.36                                                          R11 = (Stop)                                                                            D11 = 2.70                                                          R12 = -26.70                                                                            D12 = 2.42   N7 = 1.80518                                                                              ν7 = 25.4                               R13 = 65.44                                                                             D13 = 1.00                                                          R14 = -59.75                                                                            D14 = 1.00   N8 = 1.65159                                                                              ν8 = 58.5                               R15 = 27.74                                                                             D15 = 5.80   N9 = 1.80400                                                                              ν9 = 46.6                               R16 = -14.65                                                                            D16 = Variable                                                      R17 = -25.00                                                                            D17 = 2.49   N10 = 1.84665                                                                             ν10 = 23.8                              R18 = -17.35                                                                            D18 = 0.40                                                          R19 = -23.62                                                                            D19 = 1.30   N11 = 1.69679                                                                             ν11 = 55.5                              R20 = -62.18                                                                            D20 = 4.00                                                          R21 = -16.00                                                                            D21 = 1.99   N12 = 1.77249                                                                             ν12 = 49.6                              R22 = -170.60                                                                 ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R12: K = -6.839 × 10.sup.-1                                                                A = 0        B = -1.773 × 10.sup.-4                       C = 2.030 × 10.sup.-7                                                                 D = -4.483 × 10.sup.-8                                                               E = 0                                         R16: K = -2.383    A = 0        B = -1.044 × 10.sup.-4                       C = 1.158 × 10.sup.-7                                                                 D = -1.694 × 10.sup.-9                                                               E = 0                                         ______________________________________                                        Variable    Focal Length                                                      Separations 29.47        51.94  80.02                                         ______________________________________                                        D6          12.01        11.00  0.80                                          D16         8.72         2.56   2.48                                          ______________________________________                                    

                  TABLE 1                                                         ______________________________________                                                 Numerical Example                                                    Factor     1        2      3      4    5                                      ______________________________________                                        Hiw        4.11     4.01   4.68   4.83 3.66                                   |Hbiw|                                                                 1.87     0.84   3.23   3.89 3.07                                   Hit        4.13     4.02   4.27   4.28 3.74                                   |Hbit|                                                                 1.01     0.67   1.96   2.18 1.52                                   Fw/f1      0.35     0.51   0.43   0.44 0.53                                   Fw/f2      1.35     1.36   1.37   1.38 1.30                                   Fw/f3      1.18     1.12   1.45   1.55 1.40                                   f3(1-β3w)/Fw                                                                        0.32     0.32   0.32   0.32 0.29                                   β3w   1.38     1.29   1.47   1.49 1.41                                   |β2w|                                                             0.26     0.39   0.29   0.29 0.37                                   ______________________________________                                    

According to the invention, as applied to the zoom lens having aplurality of lens groups, for example, three lens groups in total, towhich all the constituent lenses are divided, an appropriate one of thelens groups and an appropriate lens surface therein are selected tointroduce an aspheric surface. This leads to a possibility of improvingthe compact form of the entire lens system in such a manner that thoseaberrations which become serious when the maximum angle of field widensand the range increases are all corrected well. Thus, a wide-angle highrange zoom lens having high optical performance throughout the entirezooming range can be achieved.

FIG. 27 to FIG. 31 are longitudinal section views of numerical examples6 to 10 of zoom lenses of the invention in the wide-angle end,respectively. FIG. 32A to FIG. 64D show the aberrations of the numericalexamples 5 to 10 of the invention. FIGS. 65(A) and 65(B) are schematicdiagrams to explain the paraxial refractive power arrangement of thezoom lens with the focusing lens unit of the invention shown.

In these drawings, L1 is the first lens group having a plurality of lensunits L1i (where i=1 to 5 inclusive) and whose overall refractive poweris positive, and L2 is the second lens group whose overall refractivepower is negative. The second lens group has two lens units, of whichthe first lens unit L21 is of negative refractive power and the secondlens unit L22 is of negative refractive power. SP is the stop. IP is theimage plane. The arrows indicate the directions in which the lens groupor units axially move when zooming from the wide-angle end to thetelephoto end. In more detail, zooming is performed by varying theseparations between each adjacent two of all the lens units in the firstlens group L1 and between the first lens group L1 and the second lensgroup L2.

In the numerical examples 6 to 10 of the present embodiment, as shown inFIGS. 65(A) and 65(B), focusing is performed by varying the separationbetween the lens unit L21 and the lens unit L22. Specifically speaking,while the lens unit L22 is fixed, the lens unit L21 moves toward theimage side as shown in FIG. 65(B). Another method is to move both of thelens unit L21 and the lens unit L22 toward the image side, whilesimultaneously varying their separation as shown in FIG. 65(A). Thesetwo focusing methods are selectively employed one at a zoom lens.

In general, the zoom lenses for photographic cameras or video camerashave their exit pupil on the object side of the image plane (filmsurface). For this reason, the lens unit L22 gets a larger effectivediameter than the lens unit L21 does. The focal length fL2 of the secondlens group can be defined as a function of the focal lengths fL21 andfL22 of the lens unit L21 and the lens unit L22, respectively, and theprincipal point interval, e, of the lens unit L21 and and the lens unitL22 by the following equation:

    fL2=fL21·fL22/(fL21+fL22-e)                       (a)

On this account, in the numerical examples 6 to 10, when focusing down,the lens unit L21 and the lens unit L22 are made to axially move in suchrelation as to decrease their separation. In turn, this weakens thenegative refractive power of the second lens group L2, thereby reducingthe total focusing movement of the lens units L21 and L22 from that whenthe second lens group L2 does it alone. By this, the requirements ofshortening the total length of the complete lens and of reducing therequired drive torque for focusing are fulfilled simultaneously.Particularly in the case that when focusing, the lens unit L22 is fixedand the lens unit L21 of smaller diameter than that of the lens unit L22is moved toward the image side, there are merits that the focusingmovement requires a lower torque and that the focusing mechanism becomessimpler in structure.

In the numerical examples 6 to 10, by employing such a zoom lensconfiguration and such a focusing lens configuration, the maximum angleof field is widened and the range is increased to 2.5 to 3.5 in the zoomratio with the limitation of the total length of the complete lens to aminimum, while still permitting the variation of aberrations withzooming to be corrected well. Thus, a high optical performance isobtained throughout the entire zooming range and throughout the entirefocusing range.

To further improve the stability of aberration correction for a higheroptical performance throughout the entire zooming range and throughoutthe entire focusing range, or to assist in constructing the lensmounting mechanism in a suitable form, the invention sets forthadditional features or conditions. So, it is recommended to satisfy atleast one of the following conditions:

(2-1) Letting the focal length for the wide-angle end of the first lensgroup L1 be denoted by fL1W, the shortest focal length of the entiresystem by fW, the focal lengths of the first lens unit L21 and thesecond lens group L2 by fL21 and fL2, respectively, and the lateralmagnification for the wide-angle end of the second lens group L2 byβL2W, the following conditions are satisfied:

    0.5<|fL2/fW|<0.95                        (11)

    0.25<fL2/fL21<0.9                                          (12)

    1.2<βL2W<1.85                                         (13)

    0.5<fL1W/fW<0.95                                           (14)

The inequalities (11) are concerned with the the ratio of the negativerefractive power of the lens group L2 including the focusing lens unitsto the shortest focal length of the entire system and have an aimchiefly to obtain the predetermined value of the zoom ratio, while stillpermitting a shortening of the total length of the complete lens to beachieved. When the upper limit of the condition (11) is exceeded, asthis means that the second lens group L2 has too weak a negativeretractive power, the zooming movements of all the lens units withinclusion of the second lens group L2 increase to obtain an equivalentzoom ratio. Therefore, the total length of the entire lens systemincreases greatly.

When the lower limit of the condition (11) is exceeded, the lens systemas a whole takes a strong form of the telephoto type, because thenegative refractive power of the second lens group L2 is too strong.Therefore, it becomes difficult to obtain the back focal distance ofplus sign. Moreover, large off-axial aberrations are produced, which aredifficult to correct well.

The inequalities (12) are concerned with the ratio of the negativerefractive power of the second lens group L2 to the negative refractivepower of the lens unit L21 as the focusing lens included in that groupand have an aim chiefly to minimize the size of the lens system in sucha manner as to suppress well the variation of aberrations with focusing.

When the lens unit L21 as the focusing lens has weaker negativerefractive power beyond the upper limit of the condition (12), the totalfocusing movement of the lens unit L21 increases unduly greatly. Forthis reason, to avoid occurrence of its mechanical interference with theother lens unit L22 in the second lens group L2, the air separationbetween the lens unit L21 and the lens unit L22 must be previously takenwider. As a result, the lens system gets longer in size.

When the lower limit of the condition (12) is exceeded, as this meansthat the lens unit L21 as the focusing lens has too strong a refractivepower, the range of variation of off-axial aberrations with focusingincreases and, at the same time, the divergence of the off-axial lightbeam is strengthened as it passes through the lens unit L21. Therefore,the outer diameter of the lens unit L22 increases objectionably.

The inequalities (13) are concerned with the lateral magnification forthe wide-angle end of the second lens group L2 and have an aim chieflyto shorten the total length of the complete lens and keep the opticalperformance in good balance. Now, letting the shortest focal length ofthe entire system be denoted by fW and the focal length for thewide-angle end of the first lens group L1 by fL1W, the equation for theshortest focal length fW is expressed by

    fW=fL1W·βL2W                                 (b)

When the second lens group L2 has a greater lateral magnification beyondthe upper limit of the condition (13), as is understandable from theequation (b), widening of the maximum angle of field requires that thepositive refractive power of the first lens group L1 be strengthened.Therefore, large spherical aberration is produced as is under-corrected,which is difficult to correct by the second lens group L2.

When the second lens group L2 has a smaller lateral magnification beyondthe lower limit of the condition (13), as the paraxial back focaldistance for the wide-angle end, βfW, of the lens system is derived fromthe following equation:

    βfW=fL2·(1-βL2W)

it becomes difficult to keep the back focal distance positive. Moreover,to hold an equivalent ratio of the corner illumination to the centerone, the outer diameter of the second lens group L2 increasesobjectionably.

The inequalities (14) are concerned with the ratio of the overallrefractive power for the wide-angle end of the multi-unit lens group L1to the shortest focal length of the entire system and have an aimchiefly to minimize the size of the entire lens system in such a manneras to well correct aberrations.

When the upper limit of the condition (14) is exceeded, as this meansthat the first lens group has too weak a refractive power, the airseparation between the lens group L1 and the lens group L2 must bepreviously taken wider to obtain an equivalent longest focal length.Therefore, the entire lens system increases in size objectionably.

When the lower limit of the condition (14) is exceeded, as this meansthat the first lens group L1 has too strong a positive refractive power,the entire lens system takes a strong form of the telephoto type.Therefore, it becomes difficult to keep the back focal distancepositive. Moreover, strong under-correction of spherical aberrationresults, which is difficult to correct by the second lens group L2.

To achieve further improvements of the correction of aberrations withthe limitation of the size of the entire lens system to a minimum, it isrecommended in the invention to narrow the ranges given by theinequalities of conditions (11) to (14) by altering their upper andlower limits as follows:

    0.6<|fL2/fW|<0.8                         (11a)

    0.35<fL2/fL21<0.7                                          (12a)

    1.3<βL2W<1.6                                          (13a)

    0.6<fL1W/fW<0.8                                            (14a)

(2-2) The lens unit L21 has a negative lens having a concave surfacefacing the image side and the lens unit L22 has a negative lens having aconcave surface facing the object side. By providing the negative lensesof such forms in the two lens units of the second lens group L2, goodbalance of spherical aberration and off-axial aberrations is maintainedstable throughout the entire focusing range. A higher performanceoptical system is thus achieved.

(2-3) The second lens group L2 is made up by introducing at least onepositive lens, so that good correction of chromatic aberrations ismaintained stable throughout the entire zooming range. In this case, itis desirable that the Abbe number ν_(p) of the material of theintroduced positive lens lies within the following range:

    ν.sub.p <35

(2-4) To introduce an aspheric surface into each of the lens groups ispreferable from the standpoint of aberration correction.

(2-5) When zooming, two or more of the lens units may otherwise be movedin unison. If so, an advantage of simplifying the structure ofconstruction of the lens barrel is produced.

(2-6) The stop SP is better arranged on zooming to axially move eitherin differential relation to, or in unison with, the lens units.According to this, despite the entrance pupil moving with zooming, thestop can be kept near to it. The curvature of field can thus beprevented from changing as the aperture opening decreases in size.

The characteristic features of the form and the construction andarrangement of the constituent lenses of each of the numerical examples6 to 10 are described below.

A zoom lens of the numerical example 6 shown in FIG. 27 includes, fromfront to rear, a first lens group L1 of positive refractive power whichhas a lens unit L11 of positive refractive power comprising a positivelens in the form of a singlet having a convex surface facing the objectside, a lens unit L12 of negative refractive power comprising a negativelens in the form of a singlet of which both surfaces are concave, a lensunit L13 comprising three lenses in the form of a triplet consisting ofa positive lens having a convex surface facing the object side, apositive lens of which both surfaces are convex, and a negative lens ofmeniscus shape convex toward the image side cemented together at theiradjoining surfaces, and whose overall refractive power is positive, astop SP, a lens unit L14 of negative refractive power comprising anegative lens in the form of a singlet of meniscus shape convex towardthe image side, and a lens unit L15 of positive refractive powercomprising a positive lens in the form of a singlet lens having a convexsurface facing the image side, totaling five lens units.

A second lens group L2 of negative refractive power is further included,which has a lens unit L21 comprising a doublet consisting of a negativelens of which both surfaces are concave and a positive lens of meniscusshape convex toward the object side cemented at their adjoining surfacesand whose overall refractive power is negative, and a lens unit L22 ofnegative refractive power comprising a negative lens in the form of asinglet having a concave surface facing the object side, totaling twolens units.

When zooming from the wide-angle end to the telephoto end, the lensunits axially move in differential relation like the arrows such thatthe separation between the lens unit L11 and the lens unit L12, theseparation between the lens unit L13 and the lens unit L14 and theseparation between the lens unit L14 and the lens unit L15 increase,while the separation between the lens unit L12 and the lens unit L13 andthe separation between the lens group L1 and the lens group L2 decrease.Focusing is performed by the lens unit L21.

A zoom lens of numerical example 7 shown in FIG. 28 includes, from frontto rear, a first lens group L1 of positive refractive power which has alens unit L11 of positive refractive power comprising a positive lens inthe form of a singlet having a convex surface facing the object side, alens unit L12 of negative refractive power comprising a negative lens inthe form of a singlet of which both surfaces are concave, a lens unitL13 comprising three lenses in the form of a triplet consisting of apositive lens having a convex surface facing the object side, a positivelens of which both surfaces are convex and a negative lens of meniscusshape convex toward the image side cemented together and whose overallrefractive power is positive, a stop SP, a lens unit L14 of negativerefractive power comprising a negative lens in the form of a singlet ofmeniscus shape convex toward the image side, and a lens unit L15 ofpositive refractive power comprising a positive lens in the form of asinglet having a convex surface facing the image side, totaling fivelens units.

A second lens group L2 of negative refractive power is further included,which has a lens unit L21 of negative refractive power comprising anegative lens in the form of a singlet of which both surfaces areconcave and a lens unit L22 of negative refractive power comprising apositive lens of meniscus shape convex toward the object side and anegative lens having a concave surface facing the object side, totalingtwo lens units.

When zooming from the wide-angle end to the telephoto end, the lensunits axially move in differential relation like the arrows such thatthe separation between the lens unit L11 and the lens unit L12, theseparation between the lens unit L13 and the lens unit L14 and theseparation between the lens unit L14 and the lens unit L15 increase,while the separation between the lens unit L12 and the lens unit L13 andthe separation between the lens group L1 and the lens group L2 decrease.Focusing is performed by the lens unit L21.

A zoom lens of numerical example 8 shown in FIG. 29 includes, from frontto rear, a first lens group L1 of positive refractive power which has alens unit L11 comprising a negative lens of which both surfaces areconcave and a positive lens having a convex surface facing the objectside, a lens unit L12 comprising three lenses in the form of a tripletconsisting of a positive lens having a convex surface facing the objectside, a positive lens of which both surfaces are convex and a negativelens of meniscus shape convex toward image side cemented together andwhose overall refractive power is positive, a stop SP, a lens unit L13of negative refractive power comprising a negative lens in the form of asinglet of meniscus shape convex toward the image side, and a lens unitL14 of positive refractive power comprising a positive lens in the formof a singlet having a convex surface facing the image side, totalingfour lens units.

A second lens group L2 of negative refractive power is further included,which has a lens unit L21 of negative refractive power comprising anegative lens in the form of a singlet of which both surfaces areconcave and a lens unit L22 of negative refractive power consisting of apositive lens of meniscus shape convex toward the object side and anegative lens of which both surfaces are concave, totaling two lensunits.

When zooming from the wide-angle end to the telephoto end, the lensunits axially move in differential relation like the arrows such thatthe separation between the lens unit L12 and the lens unit L13, theseparation between the lens unit L13 and the lens unit L14 increase,while the separation between the lens unit L11 and the lens unit L12 andthe separation between the lens group L1 and the lens group L2 decrease.Focusing is performed by the lens unit L21.

A zoom lens of numerical example 9 shown in FIG. 30 includes, from frontto rear, a first lens group L1 of positive refractive power which has alens unit L11 of negative refractive power comprising a doubletconsisting of a positive lens having a convex surface facing the imageside and a negative lens of which both surfaces are concave cementedtogether and a positive lens having a convex surface facing the objectside, a lens unit L12 comprising three lenses in the form of a tripletconsisting of a negative lens of meniscus shape convex toward the objectside, a positive lens of which both surfaces are convex and a negativelens of meniscus shape convex toward the image side cemented togetherand whose overall refractive power is positive, a stop SP, a lens unitL13 of positive refractive power comprising a negative lens of meniscusshape convex toward the image side and a doublet consisting of anegative lens of which both surfaces are concave and a positive lens ofwhich both surfaces are convex, totaling three lens units.

A second lens group L2 is further included, which has a lens unit L21 ofnegative refractive power comprising a negative lens in the form of asinglet of which both surfaces are concave and a lens unit L22comprising a positive lens of meniscus shape convex toward the objectside and a negative lens having a concave surface facing the objectside, totaling two lens units.

When zooming from the wide-angle end to the telephoto end, the lensunits axially move in differential relation like the arrows such thatthe separation between the lens unit L12 and the lens unit L13increases, while the separation between the lens unit L11 and the lensunit L12 and the separation between the lens group L1 and the lens groupL2 decrease. Focusing is performed by the lens unit L21.

A zoom lens of numerical example 10 shown in FIG. 31 includes, fromfront to rear, a first lens group L1 of positive refractive power whichhas a lens unit L11 of positive refractive power comprising a negativelens of meniscus shape convex toward the object side and a positive lenshaving a convex surface facing the object side, a stop SP and a lensunit L12 of positive refractive power comprising a doublet consisting ofa negative lens having a concave surface facing the object side and apositive lens cemented together and a doublet consisting of a negativelens of meniscus shape convex toward the object side and a positive lensof which both surfaces are convex cemented together, totaling two lensunits.

A lens group L2 of negative refractive power is further included, whichhas a lens unit L21 in the form of a doublet consisting of a positivelens of meniscus shape convex toward the image side and a negative lensof which both surfaces are concave cemented together and whose overallrefractive power is negative and a lens unit L22 of negative refractivepower comprising a negative lens in the form of a singlet of which bothsurfaces are concave, totaling two lens units.

When zooming from the wide-angle end to the telephoto end, the lensunits axially move in differential relation like the arrows such thatthe separation between the lens unit L11 and the lens unit L12increases, while the separation between the lens group L1 and the lensgroup L2 decreases. Focusing is performed by the lens unit L21.

Next, the numerical data for the numerical examples 6 to 10 of theinvention are shown in the following tables, wherein Ri is the radius ofcurvature of the i-th lens surface, when counted from the object side,Di is the i-th lens thickness or air separation, when counted from theobject side, and Ni and νi are respectively the refractive index andAbbe number of the glass of the i-th lens element, when counted from theobject side.

The values of the factors in the before-described conditions for thenumerical examples 6 to 10 are listed in Table-2.

The shape of the aspheric surface is expressed in the coordinates withan X axis in the axial direction and an H axis in the perpendiculardirection to the optical axis, the direction in which light advancesbeing taken as positive, by the following equation: ##EQU2## where R isthe radius of the osculating sphere, and K, A, A, B, C, D and E are theaspheric coefficients.

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.86 - 101.39  Fno. = 4.33 - 9.06  2ω = 73.7° -             24.1°                                                                  ______________________________________                                        R1 = 53.81                                                                              D1 = 2.40    N1 = 1.84665                                                                              ν1 = 23.8                               R2 = 1346.34                                                                            D2 = Variable                                                       R3 = -48.28                                                                             D3 = 1.20    N2 = 1.67790                                                                              ν2 = 54.9                               R4 = 20.69                                                                              D4 = Variable                                                       R5 = 14.96                                                                              D5 = 1.80    N3 = 1.80609                                                                              ν3 = 41.0                               R6 = 17.17                                                                              D6 = 3.30    N4 = 1.58913                                                                              ν4 = 61.2                               R7 = -20.02                                                                             D7 = 1.00    N5 = 1.84665                                                                              ν5 = 23.8                               R8 = -48.60                                                                             D8 = Variable                                                       R9 = Stop D9 = 2.00                                                           R10 = -20.16                                                                            D10 = 1.20   N6 = 1.80518                                                                              ν6 = 25.4                               R11 = -63.02                                                                            D11 = Variable                                                      R12 = 283.18                                                                            D12 = 4.90   N7 = 1.73077                                                                              ν7 = 40.6                               R13 = -14.50                                                                            D13 = Variable                                                      R14 = -86.38                                                                            D14 = 1.20   N8 = 1.77249                                                                              ν8 = 49.6                               R15 = 69.98                                                                             D15 = 2.20   N9 = 1.69894                                                                              ν9 = 30.1                               R16 = 86.43                                                                             D16 = 6.47                                                          R17 = -27.85                                                                            D17 = 1.50   N10 = 1.74319                                                                             ν10 = 49.3                              R18 = -4518.62                                                                ______________________________________                                        Variable    Focal Length                                                      Separations 28.86        54.10  101.39                                        ______________________________________                                        D2          2.58         2.02   2.92                                          D4          8.10         5.59   0.99                                          D8          3.19         5.94   6.94                                          D11         3.31         5.68   7.36                                          D13         7.73         2.31   0.99                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R4:  K = 2.482 × 10.sup.-1                                                                 A = 0        B = -1.870 × 10.sup.-5                       C = -4.346 × 10.sup.-8                                                                D = -2.257 × 10.sup.-10                                                              E = 0                                         R10: K = 4.04329   A = 0        B = -6.210 × 10.sup.-5                       C = -2.925 × 10.sup.-7                                                                D = -8.844 × 10.sup.-9                                                               E = 0                                         R13: K = -2.81235  A = 0        B = -8.961 × 10.sup.-5                       C = 2.350 × 10.sup.-7                                                                 D = -6.170 × 10.sup.-10                                                              E = 0                                         ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.87 - 100.96  Fno. = 4.33 - 9.06  2ω = 73.7° -             24.2°                                                                  ______________________________________                                        R1 = 53.62                                                                              D1 = 2.40    N1 = 1.84665                                                                              ν1 = 23.8                               R2 = 2359.07                                                                            D2 = Variable                                                       R3 = -47.19                                                                             D3 = 1.20    N2 = 1.67790                                                                              ν2 = 54.9                               R4 = 20.14                                                                              D4 = Variable                                                       R5 = 15.03                                                                              D5 = 1.80    N3 = 1.80609                                                                              ν3 = 41.0                               R6 = 17.51                                                                              D6 = 3.30    N4 = 1.58913                                                                              ν4 = 61.2                               R7 = -18.42                                                                             D7 = 1.00    N5 = 1.84665                                                                              ν5 = 23.8                               R8 = -37.77                                                                             D8 = Variable                                                       R9 = Stop D9 = 2.00                                                           R10 = -19.59                                                                            D10 = 1.20   N6 = 1.80518                                                                              ν6 = 25.4                               R11 = -103.12                                                                           D11 = Variable                                                      R12 = 158.88                                                                            D12 = 4.90   N7 = 1.73077                                                                              ν7 = 40.6                               R13 = -14.29                                                                            D13 = Variable                                                      R14 = -74.61                                                                            D14 = 1.20   N8 = 1.77249                                                                              ν8 = 49.6                               R15 = 48.59                                                                             D15 = 2.93                                                          R16 = 62.86                                                                             D16 = 2.50   N9 = 1.69894                                                                              ν9 = 30.1                               R17 = 155.09                                                                            D17 = 4.03                                                          R18 = -30.71                                                                            D18 = 1.50   N10 = 1.74319                                                                             ν10 = 49.3                              R19 = -2469.17                                                                ______________________________________                                        Variable    Focal Length                                                      Separations 28.87        53.99  100.96                                        ______________________________________                                        D2          2.60         1.78   3.17                                          D4          8.13         6.00   0.99                                          D8          3.11         5.62   6.81                                          D11         3.14         5.32   7.05                                          D13         7.90         2.07   0.99                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R4:  K = 2.354 × 10.sup.-1                                                                 A = 0        B = -1.566 × 10.sup.-5                       C = -5.4231 × 10.sup.-8                                                               D = -1.294 × 10.sup.-10                                                              E = 0                                         R10: K = 3.68674   A = 0        B = -6.371 × 10.sup.-5                       C = -3.434 × 10.sup.-7                                                                D = -1.145 × 10.sup.-8                                                               E = 0                                         R13: K = -2.86420  A = 0        B = -9.380 × 10.sup.-5                       C = 2.964 × 10.sup.-7                                                                 D = -8.088 × 10.sup.-10                                                              E = 0                                         ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.84 - 102.90  Fno. = 4.33 - 9.06  2ω = 73.8° -             23.8°                                                                  ______________________________________                                        R1 = -74.16                                                                              D1 = 1.10   N1 = 1.77249                                                                             ν1 = 49.6                                R2 = 20.25 D2 = 1.12                                                          R3 = 26.77 D3 = 2.20   N2 = 1.84665                                                                             ν2 = 23.8                                R4 = 62.61 D4 = Variable                                                      R5 = 14.84 D5 = 1.80   N3 = 1.80609                                                                             ν3 = 41.0                                R6 = 17.26 D6 = 4.10   N4 = 1.58913                                                                             ν4 = 61.2                                R7 = -19.94                                                                              D7 = 1.00   N5 = 1.84665                                                                             ν5 = 23.8                                R8 = -39.89                                                                              D8 = Variable                                                      R9 = Stop  D9 = 2.00                                                          R10 = -19.68                                                                             D10 = 1.20  N6 = 1.80518                                                                             ν6 = 25.4                                R11 = -75.03                                                                             D11 = Variable                                                     R12 = -5905.34                                                                           D12 = 4.30  N7 = 1.73077                                                                             ν7 = 40.6                                R13 = -15.04                                                                             D13 = Variable                                                     R14 = -55.39                                                                             D14 = 1.20  N8 = 1.77249                                                                             ν8 = 49.6                                R15 = 39.93                                                                              D15 = 2.29                                                         R16 = 37.23                                                                              D16 = 3.00  N9 = 1.69894                                                                             ν9 = 30.1                                R17 = 152.45                                                                             D17 = 3.00                                                         R18 = -41.23                                                                             D18 = 1.50  N10 = 1.77249                                                                            ν10 = 49.6                               R19 = 91.48                                                                   ______________________________________                                        Variable    Focal Length                                                      Separations 28.84        54.48  102.90                                        ______________________________________                                        D4          8.25         5.09   1.03                                          D8          3.89         6.21   7.23                                          D11         2.44         4.74   6.03                                          D13         7.56         2.56   0.97                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R1:  K = -1.350 × 10.sup.-1                                                               A = 0        B = -3.125 × 10.sup.-6                        C = -2.570 × 10.sup.-8                                                               D = -3.970 × 10.sup.-11                                                              E = 0                                          R10: K = 3.77316  A = 0        B = -8.280 × 10.sup.-5                        C = -4.129 × 10.sup.-7                                                               D = -1.164 × 10.sup.-8                                                               E = 0                                          R13: K = -2.97836 A = 0        B = -1.000 × 10.sup.-4                        C = 2.620 × 10.sup.-7                                                                D = -1.139 × 10.sup.-9                                                               E = 0                                          ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 29.18 - 88.82  Fno. = 3.75 - 9.00  2ω = 73.1° - 27.4.degr    ee.                                                                           ______________________________________                                        R1 = -205.07                                                                             D1 = 2.80   N1 = 1.51633                                                                             ν1 = 64.2                                R2 = -42.00                                                                              D2 = 1.20   N2 = 1.80400                                                                             ν2 = 46.6                                R3 = 16.58 D3 = 2.34                                                          R4 = 20.25 D4 = 4.20   N3 = 1.84665                                                                             ν3 = 23.8                                R5 = 200.06                                                                              D5 = Variable                                                      R6 = 17.29 D6 = 0.90   N4 = 1.84665                                                                             ν4 = 23.8                                R7 = 11.68 D7 = 5.20   N5 = 1.48749                                                                             ν5 = 70.2                                R8 = -20.58                                                                              D8 = 0.90   N6 = 1.84665                                                                             ν6 = 23.8                                R9 = -29.72                                                                              D9 = Variable                                                      R10 = Stop D10 = 2.00                                                         R11 = -24.06                                                                             D11 = 2.00  N7 = 1.80518                                                                             ν7 = 25.4                                R12 = -35.93                                                                             D12 = 0.73                                                         R13 = -22.89                                                                             D13 = 1.30  N8 = 1.65159                                                                             ν8 = 58.5                                R14 = 73.08                                                                              D14 = 4.00  N9 = 1.77249                                                                             ν9 = 49.6                                R15 = -13.31                                                                             D15 = Variable                                                     R16 = -66.06                                                                             D16 = 1.20  N10 = 1.77249                                                                            ν10 = 49.6                               R17 = 50.01                                                                              D17 = 2.26                                                         R18 = 41.36                                                                              D18 = 2.90  N11 = 1.69894                                                                            ν11 = 30.1                               R19 = 221.99                                                                             D19 = 5.02                                                         R20 = -23.64                                                                             D20 = 1.50  N12 = 1.77249                                                                            ν12 = 49.6                               R21 = -173.10                                                                 ______________________________________                                        Variable    Focal Length                                                      Separations 29.18        50.91  88.82                                         ______________________________________                                        D5          7.60         5.46   2.62                                          D9          5.30         10.56  13.45                                         D15         11.94        5.16   0.99                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R3:  K = 2.216 × 10.sup.-1                                                                A = 0        B = -1.254 × 10.sup.-5                        C = 2.253 × 10.sup.-8                                                                D = -5.639 × 10.sup.-10                                                              E = 0                                          R11: K = 6.26902  A = 0        B = -6.294 × 10.sup.-5                        C = -3.412 × 10.sup.-7                                                               D = -9.389 × 10.sup.-9                                                               E = 0                                          R15: K = -2.72205 A = 0        B = -1.510 × 10.sup.-4                        C = 1.387 × 10.sup.-7                                                                D = -3.214 × 10.sup.9                                                                E = 0                                          ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 39.67 - 101.24  Fno. = 4.00 - 7.80  2ω = 57.2° -             24.1°                                                                  ______________________________________                                        R1 = 48.86 D1 = 1.20   N1 = 1.84665                                                                             ν1 = 23.9                                R2 = 35.56 D2 = 0.30                                                          R3 = 24.35 D3 = 2.80   N2 = 1.48749                                                                             ν2 = 70.2                                R4 = 166.60                                                                              D4 = Variable                                                      R5 = Stop  D5 = 2.00                                                          R6 = -17.23                                                                              D6 = 1.20   N3 = 1.48749                                                                             ν3 = 70.2                                R7 = -168.98                                                                             D7 = 8.49   N4 = 1.80517                                                                             ν4 = 25.4                                R8 = -33.56                                                                              D8 = 4.74                                                          R9 = 36.00 D9 = 1.10   N5 = 1.84665                                                                             ν5 = 23.9                                R10 = 17.05                                                                              D10 = 5.30  N6 = 1.58312                                                                             ν6 = 59.4                                R11 = -30.35                                                                             D11 = Variable                                                     R12 = -49.04                                                                             D12 = 3.40  N7 = 1.69894                                                                             ν7 = 30.1                                R13 = -25.21                                                                             D13 = 1.10  N8 = 1.58913                                                                             ν8 = 61.2                                R14 = 85.79                                                                              D14 = 8.00                                                         R15 = -65.34                                                                             D15 = 1.50  N9 = 1.63853                                                                             ν9 = 55.4                                R16 = 169.94                                                                  ______________________________________                                        Variable    Focal Length                                                      Separations 39.67        64.41  101.24                                        ______________________________________                                        D4          0.90         12.13  21.64                                         D11         15.70        7.21   1.19                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R11: K = -3.015 × 10.sup.-2                                                               A = 0        B = 8.045 × 10.sup.-6                         C = -1.134 × 10.sup.-7                                                               D = 1.843 × 10.sup.-9                                                                E = -1.305 × 10.sup.-11                  ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                               Numerial Example                                                       Factor   6         7      8       9    10                                     ______________________________________                                        |fL2/fW|                                                             0.71      0.73   0.69    0.76 0.67                                   fL2/fL21 0.38      0.55   0.66    0.61 0.51                                   βL2W                                                                              1.46      1.47   1.53    1.40 1.32                                   fL1W/fW  0.69      0.68   0.65    0.71 0.76                                   ______________________________________                                    

It will be appreciated from the foregoing that, according to theinvention, in the second embodiment thereof, the rear focus type isused, and the paraxial refractive power arrangement of the entiresystem, the zooming movements of all the lens groups and units, andother rules of lens design are made appropriate to maintain goodstability of aberration correction throughout the entire zooming rangeand throughout the entire focusing range, thus achieving a wide-anglehigh range zoom lens.

FIG. 66 to FIG. 68 are diagrams to explain the paraxial refractive powerarrangements of numerical examples 11 to 17 of zoom lenses of theinvention. FIG. 69 to FIG. 75 are longitudinal section views of the zoomlenses of numerical examples 11 to 17 of the invention, respectively.FIG. 76A to FIG. 96D show the aberrations of the numerical examples 11to 17 of the invention.

In these drawings, L1 is the first lens group of negative refractivepower, L2 is the second lens group of positive refractive power, L3 isthe third lens group of positive refractive power and L4 is the fourthlens group of negative refractive power. SP is the stop, SPP is thefixed stop, and IP is the image plane.

In the numerical examples 11 to 17 of the present embodiment, whenzooming from the wide-angle end to the telephoto end, all the lensgroups are made to axially move toward the object side in such relationthat the separation between the first lens group L1 and the second lensgroup L2 and the separation between the third lens group L3 and thefourth lens group L4 decrease, while the separation between the secondlens group L2 and the third lens group L3 increases.

In general, the zoom lenses produce lateral chromatic aberration andspherical aberration in some regions of the zooming range. For theseaberrations to be corrected effectively, it is desirable to use as smalla number of constituent lenses in each lens group as possible. Onconsideration of the gamut of available real optical glasses, however,such an approach is very difficult to adopt. If one relies on analternative correcting method which is to increase the number ofconstituent lenses, these lenses must be so disposed as not to interferewith one another, so that a certain ratio of the corner illumination tothe central illumination is assured. In some cases, therefore, ithappens that the back focal distance is hardly kept at a predeterminedvalue. In other cases, the outer diameter of the lens system becomes toolarge, causing the complexity of structure of the operating mechanism toincrease largely.

From these reasons, the zoom lenses of the numerical examples 11 to 17are designed basically as comprising, from front to rear, a first lensgroup of negative refractive power, a second lens group of positiverefractive power, a third lens group of positive refractive power and afourth lens group of negative refractive power, totaling four lensgroups, wherein when zooming from the wide-angle end to the telephotoend, all the lens groups are made to axially move toward the object sidein such relation that the separation between the first lens group andthe second lens group and the separation between the third lens groupand the fourth lens group decrease, while the separation between thesecond lens group and the third lens group increases. Another basic ruleof design is that the whole lens system keeps a nearly symmetric formduring zooming. A good stability of optical performance is thusmaintained throughout the entire zooming range. Particularly, on thewide-angle side, the whole lens system takes the retrofocus type, thusmaking it easy to widen the angle of field.

FIG. 66 shows the variation with zooming of the paraxial refractivepower arrangement for the numerical examples 11 to 13 of the invention.The first lens group L1 is constructed with two lens units, of which thefirst lens unit L11 as viewed from the object side is of positiverefractive power, and the second lens unit L12 is of negative refractivepower. When zooming from the wide-angle end to the telephoto end, thefirst to the fourth lens groups move as described before and, at thesame time, the separation between the lens unit L11 and the lens unitL12 is made to increase. With this, when in the wide-angle end, negativedistortion is well corrected and, when in the telephoto end, thetelephoto type is formed, thus achieving improvements of the opticalperformance.

FIG. 67 shows the variation with zooming of the paraxial refractivepower arrangement for the numerical examples 14 and 15 of the invention.The first to the fourth lens groups move as described before. Inparticular, the lens system keeps the symmetric form as a whole duringzooming. With this, good optical performance is obtained when inachieving a shortening of the total length for the wide-angle end of theentire system.

FIG. 68 shows the variation with zooming of the paraxial refractivepower arrangement for the numerical examples 16 and 17 of the invention.In this instance, the first lens group L1 is constructed with a lensunit L11 of positive refractive power and a lens unit L12 of negativerefractive power, and the third lens group L3 is constructed with a lensunit L31 of negative refractive power and a lens unit L32 of positiverefractive power. When zooming from the wide-angle end to the telephotoend, the first to the fourth lens groups move as described before and,at the same time, the separation between the lens unit L11 and lens unitL12 increases, while the separation between the lens unit L31 and thelens unit L32 also increases. By this, the off-axial light beam, inparticular, is made to pass through the lens unit L32 at a fartherposition from the optical axis, thus correcting on-axial aberrations andoff-axial aberrations in good balance.

In addition, that increase of the range of variation with zooming ofchromatic aberrations which results from the increase of the zoom ratiois well corrected for high optical performance throughout the entirezooming range. To this purpose, the numerical examples 11 to 17 employat least either one of the following two features (i) and (ii),

(i) Letting the lateral magnification for the wide-angle end and thetelephoto end of the i-th lens group (where i≧2) be denoted by βiW andβiT, respectively, and putting the rate of variation of themagnification Δβi of the i-th lens group to be

    Δβi=|βiT/βiW|

the second lens group has, under the condition of Δβi>1, to have ahighest rate of variation of the magnification among all of the lensgroups of positive refractive powers and includes a triplet (lens A)consisting of either two positive lenses or two negative lenses and alens whose refractive power is of opposite sign thereto cementedtogether at their adjoining surfaces and whose overall refractive poweris positive.

(ii) The second lens group has a triplet (lens A) consisting of eithertwo positive lenses or two negative lenses and a lens whose refractivepower is of opposite sign thereto cemented together at their adjoiningsurfaces and whose overall refractive power is positive, and it is onthe image side of the second lens group that there is provided anadditional lens group B of negative refracting power (FB<0) satisfyingthe following condition:

    |FB/FW|<1.1                              (20)

where FB is the focal length of the additional lens group and FW is theshortest focal length of the entire system.

In here, the inequality of condition (20) has an aim to achieve aminimization of the size of the entire lens system with high efficiency.When this condition is violated, the zooming movements of those lensgroups of negative refractive powers which lie on the image side of thelens A-included second lens group weaken their contributions to thevariation of the focal length. Moreover, the effect of the retrofocustype weakens in the wide angle end, causing the size of the entire lenssystem to increase objectionably.

Next, the features of the lens A provided in the second lens group aredescribed. FIG. 97, FIG. 98 and FIG. 99 are concerned with the numericalexample 15 of the invention, showing the longitudinal chromaticaberration, lateral chromatic aberration and spherical aberration of 3rdorder respectively with an object at infinity. In these graphs, for eachof all lens groups in the zoom lens system, there are shown the producedamounts of these aberrations at any zooming station. The total sum ofthese amounts for each aberration is also shown.

In here, with the second lens group (L2) of positive refractive power inmind, it is understandable that its aberrations cancel the longitudinalchromatic aberration and lateral chromatic aberration the first lensgroup (L1) of negative refractive power produces and the lateralchromatic aberration and spherical aberration the fourth lens group (L4)of negative refractive power produces.

Here, as the lens A of the character described before is introduced intothe second lens group (L2), the two cemented surfaces in this lens Acooperate to well correct chromatic aberrations and sphericalaberration. In this respect, it is desirable that the front and rearcemented surfaces each are arranged to have a negative refracting power.If so, the chromatic aberrations and spherical aberration the secondlens group (L2) produce can be corrected in good balance at the imageplane of the zoom lens system.

It should be noted that, if, in the numerical examples 11 to 17 of theinvention, the cemented surfaces of the lens A were broken to create airspaces between any two of the lens surfaces, large spherical aberrationsof higher order would be produced, which are difficult to correct by theother lens surfaces. If the cemented surface were one in number, itwould become difficult to correct chromatic aberrations and sphericalaberration in good balance.

The above-described features or conditions suffice for accomplishing theobjects of the invention. However, to further improve the opticalperformance throughout the entire zooming range with the limitation ofthe size of the entire lens system to a minimum, it is recommended thatwithin the framework of the constructional features (i) and (ii)described before, at least one of the following additional features orconditions is satisfied.

(3-1) Letting the focal length of the lens group C of negativerefractive power that has the highest rate of variation of themagnification with zooming be denoted by FC, and the shortest focallength of the entire system by FW, the following condition is satisfed:

    0.5<|FC/FW|<1.2                          (21)

In here, the lens group C in FIG. 66 and FIG. 67 corresponds to thefourth lens group and, in FIG. 68, to the lens unit L31 or the fourthlens group. When the upper limit of the condition (21) is exceeded, asthis means that the lens group C has too weak a refractive power, thetotal zooming movement of the lens group C must be increased to obtainan equivalent zoom ratio. As a result, the total length of the entiresystem increases objectionably. When the refractive power of the lensgroup C is too strong beyond the lower limit, large off-axialaberrations such as coma and field curvature are produced, which aredifficult to correct well.

(3-2) To make the lens A constructed with, from front to rear, anegative lens A1 of meniscus shape convex toward the object side, apositive lens A2 of which both surfaces are convex and a negative lensA3 of meniscus shape convex toward the image side. Now, the cementedsurface between the negative lens A1 and the positive lens A2 isreferred to as "S1", and the cemented surface between the positive lensA2 and the negative lens A3 is referred to as "S2". Then, for thecemented surface S1 alone to produce an effect strong enough to correctchromatic aberrations, the cemented surface S1 must be made strong incurvature to strengthen the refracting power of the cemented surface. Ifthe curvature is merely strengthened, however, the cemented surfacefunctions in such a way that, for the axial beam, compared with theparaxial zone, influence by refraction is given to that part of the beamwhich passes the marginal zone. Therefore, spherical aberrations ofhigher order come to be produced.

For this reason, in the invention, provision is made of the two cementedsurfaces, say, the cemented surface S1 and the cemented surface S2, thusincreasing the number of those lens surfaces which have the function ofcorrecting chromatic aberrations in the lens A. Moreover, at the sametime, spherical aberration is corrected in good balance with chromaticaberrations.

(3-3) The stop is disposed on the image side of the second lens group.In the zoom lens of the invention, the off-axial beam passes through thefourth lens group at a near position to the optical axis. For thisreason, in the invention, the stop is disposed as described above tothereby well correct various off-axial aberrations produced by thefourth lens group.

(3-4) Letting the overall focal length for the wide-angle end of thefirst lens group and the second lens group be denoted by F1,2W, and theshortest focal length of the entire system by FW, the followingcondition is satisfied:

    0.8<F1,2W/FW<1.7                                           (22)

The inequalities of condition (22) have an aim to minimize the size ofthe entire lens system in such a manner that good optical performance ismaintained stable over the entire zooming range. When the upper limit ofthe condition (22) is exceeded, the refractive power of the othernegative lens group must be weakened to obtain an equivalent shortestfocal length. As a result, the total zooming movements of the fourthlens group and others increase to obtain an equivalent zoom ratio.Moreover, to avoid occurrence of mechanical interference between thelens groups, wider room must be made for the separations between any twoof the lens groups, causing the total length of the entire system toincrease objectionably.

When the lower limit is exceeded, on the other hand, as this means thatthe overall focal length F1,2W of the first lens group and the secondlens group is too short, it becomes difficult to obtain thepredetermined back focal distance. Moreover, large negative sphericalaberration is produced, which is difficult to correct well.

(3-5) For the aforesaid triplet, letting the Abbe number of the materialof the i-th lens, when counted from the object side be denoted by νAi,the radius of curvature of the i-th lens surface by RAi, and the focallength of the triplet by FA, the following conditions are satisfied:

    0.6<FA/FW<1.3                                              (23)

    20<νA2-(νA1+νA3)/2                                (24)

    0.13<(RA3+RA2)/(RA3-RA2)<0.45                              (25)

The inequalities of condition (23) are concerned with the ratio of therefractive power for the wide-angle end of the second lens group to therefractive power of the entire system. When the upper limit of thecondition (23) is exceeded, as this means that the second lens group hastoo weak a refractive power, the total zooming movement of the secondlens group increases to obtain an equivalent zoom ratio. As a result,the total length of the entire system comes to increase. Conversely,when the lower limit is exceeded, as this means that the second lensgroup has too strong a refractive power, large spherical aberrations ofhigher order and large coma are produced, which are difficult to correctwell.

The inequality of condition (24) has an aim to make the triplet (lens A)able to effectively correct the chromatic aberrations produced by thefourth lens group. When this condition is violated, it becomes difficultto correct chromatic aberrations well throughout the entire zoomingrange.

The inequalities of condition (25) are concerned with the shape factorof the lens A2 of bi-convex form in the lens A. When outside the rangeof the condition (25), the cemented surfaces can no longer correctspherical aberration and chromatic aberrations in good balance.Therefore, it becomes difficult to achieve an optical system of highimage quality over the entire zooming range.

(3-6) To make up the second lens group from two positive lenses or twonegative lenses and one lens of refractive power of opposite signthereto cemented together to form a triplet. According to this, theentire system gets a shorter total length and, at the same time, theposition of the entrance pupil can be brought toward the object side,thereby minimizing the diameter of the front lens members.

(3-7) To introduce an aspheric surface into each of the lens groupsadvantageous. According this, the optical performance can be furtherimproved.

(3-8) With regard to the zooming movements of the lens units, a methodof moving two or more of the lens units in unison may be employed.According to this, an advantage is obtained when the barrel structure isto simplify.

(3-9) For a lens system having a stop, when zooming, this stop may bemoved axially either in differential relation to, or in unison with, theother lens units than that including that stop. This makes it possiblefor the stop to take its place near the entrance pupil which moves whenzooming, giving an advantage of preventing change of field curvaturefrom occurring when the aperture opening decreases in size.

(3-10) In the present embodiment, for focusing purposes, a desired oneof those lens groups which vary their lateral magnification not pastunity during zooming may apply. If the first lens group has aconsiderably strong refractive power, it is preferred to employ themethod of moving the first lens group toward the object side, since thefocusing movement for a given object distance remains constant at anyzooming station. This leads to simplify the structure of the operatingmechanism.

If the back focal distance for the wide-angle end is sufficiently longand the refractive power of the last lens group is considerably strong,it is better to employ the method of moving the last lens group towardthe image side, as focusing is effected down. In this case, there is anadvantage of minimizing the diameter of the first lens group. Anothermethod of moving two or more of all the lens groups simultaneously maybe employed. Incidentally, if the focusing lens group includes the stop,the use of a focusing method of moving the focusing lens group whilekeeping the stop axially stationary gives an advantage that the drivetorque of the focusing control mechanism can be reduced.

(3-11) The triplet (lens A) in the second lens group may otherwise beconstructed with, from front to rear, a positive lens having a convexsurface facing the object side, a negative lens of which both surfacesare concave and a positive lens having a convex surface facing the imageside cemented together at their adjoining surfaces.

(3-12) In the present embodiment, it is good to make up the first lensgroup L1 either from a lens unit L11 comprising a positive lens in theform of a singlet and a lens unit L12 comprising either a negative lensand a positive lens, totaling two lenses, or a negative lens, totalingtwo lens units, or from one lens unit comprising a positive lens, anegative lens and a positive lens, totaling three lenses.

It is good to make up the second lens group L2 from a positive lenshaving a convex surface facing the object side, a positive lens of whichboth surfaces are convex and a negative lens of meniscus shape convextoward the image side, totaling three lenses, cemented together at theiradjoining surfaces to form a triplet.

It is good to make up the third lens group L3 either from a negativelens and a doublet consisting of a negative lens and a positive lens,totaling three lenses, or from a negative lens and a positive lens,totaling two lenses.

It is good to make up the fourth lens group L4 either from a positivelens of meniscus shape convex toward the image side and at least onenegative lens, or from a negative lens having a concave surface facingthe object side, a positive lens and a negative lens, totaling threelenses.

Next, the numerical data for the numerical examples 11 to 17 of theinvention are given in the following tables, wherein Ri is the radius ofcurvature of the i-th lens surface, when counted from the object side,Di is the i-th lens thickness or air separation, when counted from theobject side, and Ni and νi are respectively the refractive index andAbbe number of the glass of the i-th lens element, when counted from theobject side. The values of the factors in the above-described conditionsfor the numerical examples 11 to 17 are listed in Table-3.

The shape of the aspheric surface is expressed in the coordinates withan X axis in the axial direction and an H axis in the perpendiculardirection to the optical axis, the direction in which light advancesbeing taken as positive by the following equation: ##EQU3## where R isthe radius of the osculating sphere, and K, A, B, C and D are theaspheric coefficients.

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.84 - 101.06  Fno. = 4.33 - 9.00  2ω = 73.8° -             24.2°                                                                  ______________________________________                                        R1 = 89.26 D1 = 3.30   N1 = 1.51633                                                                             ν1 = 64.2                                R2 = -73.65                                                                              D2 = Variable                                                      R3 = -43.75                                                                              D3 = 1.20   N2 = 1.80400                                                                             ν2 = 46.6                                R4 = 18.20 D4 = 1.66                                                          R5 = 21.94 D5 = 3.00   N3 = 1.84665                                                                             ν3 = 23.8                                R6 = 101.94                                                                              D6 = Variable                                                      R7 = 16.54 D7 = 0.90   N4 = 1.84665                                                                             ν4 = 23.8                                R8 = 11.47 D8 = 4.80   N5 = 1.48749                                                                             ν5 = 70.2                                R9 = -22.49                                                                              D9 = 1.10   N6 = 1.84665                                                                             ν6 = 23.8                                R10 = -25.48                                                                             D10 = 0.80                                                         R11 = Stop D11 = 1.50                                                         R12 = -24.98                                                                             D12 = 1.60  N7 = 1.80518                                                                             ν7 = 25.4                                R13 = -47.12                                                                             D13 = 1.60                                                         R14 = -35.21                                                                             D14 = 1.90  N8 = 1.65159                                                                             ν8 = 58.5                                R15 = -88.13                                                                             D15 = 3.50  N9 = 1.77249                                                                             ν9 = 49.6                                R16 = -15.90                                                                             D16 = Variable                                                     R17 = -28.58                                                                             D17 = 2.10  N10 = 1.84665                                                                            ν10 = 23.8                               R18 = -19.52                                                                             D18 = 3.33                                                         R19 = -17.07                                                                             D19 = 1.80  N11 = 1.80400                                                                            ν11 = 46.6                               R20 = 93.48                                                                   ______________________________________                                        Variable    Focal Length                                                      Separations 28.84        53.99  101.06                                        ______________________________________                                        D2          1.28         2.10   3.02                                          D6          12.25        7.33   0.79                                          D11         1.50         3.84   6.59                                          D16         12.88        5.14   0.80                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R12:  k = 4.465    A = 0        B = -2.734 × 10.sup.-5                        C = -1.812 × 10.sup.-7                                                               D = 2.051 × 10.sup.-9                                ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.74 - 101.17  Fno. = 4.33 - 9.00  2ω = 74.0° -             24.1°                                                                  ______________________________________                                        R1 = 639.62                                                                              D1 = 2.50   N1 = 1.51633                                                                             ν1 = 64.2                                R2 = -56.09                                                                              D2 = Variable                                                      R3 = -41.05                                                                              D3 = 1.20   N2 = 1.80400                                                                             ν2 = 46.6                                R4 = 19.48 D4 = 1.64                                                          R5 = 23.15 D5 = 2.75   N3 = 1.84665                                                                             ν3 = 23.8                                R6 = 160.15                                                                              D6 = Variable                                                      R7 = 15.90 D7 = 0.90   N4 = 1.84665                                                                             ν4 = 23.8                                R8 = 11.73 D8 = 4.80   N5 = 1.48749                                                                             ν5 = 70.2                                R9 = -21.42                                                                              D9 = 0.90   N6 = 1.84665                                                                             ν6 = 23.8                                R10 = -28.84                                                                             D10 = Variable                                                     R11 = Stop D11 = 1.50                                                         R12 = -27.14                                                                             D12 = 1.60  N7 = 1.80518                                                                             ν7 = 25.4                                R13 = -48.22                                                                             D13 = 1.36                                                         R14 = -38.16                                                                             D14 = 2.00  N8 = 1.65159                                                                             ν8 = 58.5                                R15 = -26.71                                                                             D15 = 3.50  N9 = 1.77249                                                                             ν9 = 49.6                                R16 = -15.71                                                                             D16 = Variable                                                     R17 = -34.31                                                                             D17 = 2.30  N10 = 1.84665                                                                            ν10 = 23.8                               R18 = -21.84                                                                             D18 = 0.91                                                         R19 = -32.78                                                                             D19 = 1.30  N11 = 1.69679                                                                            ν11 = 55.5                               R20 = -58.16                                                                             D20 = 3.21                                                         R21 = -18.28                                                                             D21 = 1.50  N12 = 1.77249                                                                            ν12 = 49.6                               R22 = 223.19                                                                  ______________________________________                                        Variable    Focal Length                                                      Separations 28.74        53.92  101.17                                        ______________________________________                                        D2          1.05         2.26   4.97                                          D6          11.24        6.44   0.85                                          D10         1.47         4.14   6.17                                          D16         12.97        5.10   0.80                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R12:  K = 4.415    A = 0        B = -4.331 × 10.sup.-5                        C = -1.859 × 10.sup.-7                                                               D = -6.129 × 10.sup.-10                              ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.85 - 101.00  Fno. = 3.30 - 9.00  2ω = 73.7°- 24.2.degr    ee.                                                                           ______________________________________                                        R1 = 424.11                                                                              D1 = 2.40   N1 = 1.51633                                                                             ν1 = 64.2                                R2 = -60.06                                                                              D2 = Variable                                                      R3 = -38.54                                                                              D3 = 1.20   N2 = 1.80400                                                                             ν2 = 46.6                                R4 = 19.56 D4 = 1.35                                                          R5 = 21.49 D5 = 2.90   N3 = 1.84665                                                                             ν3 = 23.8                                R6 = 176.01                                                                              D6 = Variable                                                      R7 = 15.65 D7 = 0.90   N4 = 1.84665                                                                             ν4 = 23.8                                R8 = 11.27 D8 = 4.50   N5 = 1.48749                                                                             ν5 = 70.2                                R9 = -21.44                                                                              D9 = 0.90   N6 = 1.84665                                                                             ν6 = 23.8                                R10 = -29.88                                                                             D10 = Variable                                                     R11 = Stop D11 = 3.00                                                         R12 = -24.67                                                                             D12 = 2.55  N7 = 1.80518                                                                             ν7 = 25.4                                R13 = -47.29                                                                             D13 = 0.50                                                         R14 = -36.54                                                                             D14 = 1.00  N8 = 1.65159                                                                             ν8 = 58.5                                R15 = 155.75                                                                             D15 = 5.80  N9 = 1.77249                                                                             ν9 = 49.6                                R16 = -14.23                                                                             D16 = Variable                                                     R17 = -28.76                                                                             D17 = 2.30  N10 = 1.84665                                                                            ν10 = 23.8                               R18 = -20.20                                                                             D18 = 0.30                                                         R19 = -25.76                                                                             D19 = 1.30  N11 = 1.69679                                                                            ν11 = 55.5                               R20 = -80.69                                                                             D20 = 3.51                                                         R21 = -18.83                                                                             D21 = 1.50  N12 = 1.77249                                                                            ν12 = 49.6                               R22 = 431.90                                                                  ______________________________________                                        Variable    Focal Length                                                      Separations 28.85        53.98  101.00                                        ______________________________________                                        D2          1.23         2.21   5.15                                          D6          8.73         4.75   0.81                                          D10         0.79         4.84   8.26                                          D16         10.75        4.72   0.85                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R12:  K = 4.963    A = 0        B = -6.074 × 10.sup.-5                        C = -3.607 × 10.sup.-7                                                               D = -3.331 × 10.sup.-9                               R16:  K = -2.664   A = 0        B = -1.127 × 10.sup.-4                        C = 1.634 × 10.sup.-7                                                                D = -1.376 × 10.sup.-9                               ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.86 - 101.58  Fno. = 3.06 - 9.00  2ω = 73.7° -             24.1°                                                                  ______________________________________                                        R1 = 101.89                                                                              D1 = 2.85   N1 = 1.51633                                                                             ν1 = 64.2                                R2 = -61.28                                                                              D2 = 0.84                                                          R3 = -39.42                                                                              D3 = 1.20   N2 = 1.80400                                                                             ν2 = 46.6                                R4 = 17.90 D4 = 1.07                                                          R5 = 19.60 D5 = 3.35   N3 = 1.84665                                                                             ν3 = 23.8                                R6 = 95.82 D6 = Variable                                                      R7 = 16.21 D7 = 0.90   N4 = 1.84665                                                                             ν4 = 23.8                                R8 = 11.92 D8 = 4.80   N5 = 1.48749                                                                             ν5 = 70.2                                R9 = -19.85                                                                              D9 = 0.90   N6 = 1.84665                                                                             ν6 = 23.8                                R10 = -27.78                                                                             D10 = Variable                                                     R11 = Stop D11 = 3.50                                                         R12 = -26.05                                                                             D12 = 2.42  N7 = 1.80518                                                                             ν7 = 25.4                                R13 = -45.55                                                                             D13 = 0.56                                                         R14 = -34.59                                                                             D14 = 1.00  N8 = 1.65159                                                                             ν8 = 58.5                                R15 = 310.25                                                                             D15 = 5.80  N9 = 1.77249                                                                             ν9 = 49.6                                R16 = -13.79                                                                             D16 = Variable                                                     R17 = -28.83                                                                             D17 = 2.30  N10 = 1.84665                                                                            ν10 = 23.8                               R18 = -20.42                                                                             D18 = 0.24                                                         R19 = -25.56                                                                             D19 = 1.30  N11 = 1.69679                                                                            ν11 = 55.5                               R20 = -131.56                                                                            D20 = 3.53                                                         R21 = -20.55                                                                             D21 = 1.50  N12 = 1.77249                                                                            ν12 = 49.6                               R22 = 204.29                                                                  ______________________________________                                        Variable    Focal Length                                                      Separations 28.86        54.15  101.58                                        ______________________________________                                        D6          9.49         5.78   0.79                                          D10         1.56         5.74   8.67                                          D16         9.64         3.97   0.79                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R12: K = 6.017    A = 0        B = -6.890 × 10.sup.-5                        C = -6.114 × 10.sup.-7                                                               D = -4.934 × 10.sup.-9                                R16: K = -2.445   A = 0        B = -1.158 × 10.sup.-4                        C = 1.246 × 10.sup.-7                                                                D = -1.894 × 10.sup.-9                                ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.85 - 101.49  Fno. = 3.29 - 9.00  2ω = 73.7° -             24.1°                                                                  ______________________________________                                        R1 = 163.89                                                                             D1 = 3.25   N1 = 1.51633                                                                              ν1 = 64.2                                R2 = -26.34                                                                             D2 = 1.20   N2 = 1.80400                                                                              ν2 = 46.6                                R3 = 16.98                                                                              D3 = 1.21                                                           R4 = 18.93                                                                              D4 = 2.50   N3 = 1.84665                                                                              ν3 = 23.8                                R5 = 108.28                                                                             D5 = Variable                                                       R6 = 16.38                                                                              D6 = 0.90   N4 = 1.84665                                                                              ν4 = 23.8                                R7 = 11.85                                                                              D7 = 4.60   N5 = 1.48749                                                                              ν5 = 70.2                                R8 = -19.38                                                                             D8 = 0.90   N6 = 1.84665                                                                              ν6 = 23.8                                R9 = -25.91                                                                             D9 = Variable                                                       R10 = Stop                                                                              D10 = 2.00                                                          R11 = -24.61                                                                            D11 = 2.07  N7 = 1.80518                                                                              ν7 = 25.4                                R12 = -71.21                                                                            D12 = 0.48                                                          R13 = -44.54                                                                            D13 = 1.30  N8 = 1.65159                                                                              ν8 = 58.5                                R14 = -317.49                                                                           D14 = 5.00  N9 = 1.77249                                                                              ν9 = 49.6                                R15 = -13.39                                                                            D15 = Variable                                                      R16 = -33.60                                                                            D16 = 2.30  N10 = 1.84665                                                                             ν10 = 23.8                               R17 = -22.01                                                                            D17 = 0.77                                                          R18 = -25.47                                                                            D18 = 1.30  N11 = 1.69679                                                                             ν11 = 55.5                               R19 = -654.85                                                                           D19 = 4.11                                                          R20 = -18.51                                                                            D20 = 1.50  N12 = 1.77249                                                                             ν12 = 49.6                               R21 = -202.43                                                                 ______________________________________                                        Variable    Focal Length                                                      Separations 28.85        54.11  101.49                                        ______________________________________                                        D5          7.30         4.25   0.29                                          D9          1.00         5.83   8.72                                          D15         10.18        4.20   0.79                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R3:   K = 2.970 × 10.sup.-1                                                                A = 0        B = -5.495 × 10.sup.-6                        C = -1.753 × 10.sup.-8                                            R11:  K = 5.794    A = 0        B = -7.033 × 10.sup.-6                        C = -5.377 × 10.sup.-7                                                               D = -8.804 × 10.sup.-9                               R15:  K = -2.657   A = 0        B = -1.393 × 10.sup.-4                        C = 1.880 × 10.sup.-7                                                                D = -2.909 × 10.sup.-9                               ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.83 - 101.02  Fno. = 4.00 - 8.75   2ω = 73.8° -            24.2°                                                                  ______________________________________                                        R1 = 62.55                                                                              D1 = 2.40   N1 = 1.84665                                                                              ν1 = 23.8                                R2 = -704.82                                                                            D2 = Variable                                                       R3 = -79.39                                                                             D3 = 1.20   N2 = 1.77249                                                                              ν2 = 49.6                                R4 = 27.29                                                                              D4 = Variable                                                       R5 = 14.31                                                                              D5 = 0.90   N3 = 1.84665                                                                              ν3 = 23.8                                R6 = 13.95                                                                              D6 = 3.50   N4 = 1.51822                                                                              ν4 = 59.0                                R7 = -22.52                                                                             D7 = 0.90   N5 = 1.84665                                                                              ν5 = 23.8                                R8 = -40.19                                                                             D8 = 1.00                                                           R9 = Stop D9 = 4.67                                                           R10 = -17.92                                                                            D10 = 2.85  N6 = 1.80518                                                                              ν6 = 25.4                                R11 = -109.72                                                                           D11 = Variable                                                      R12 = 264.84                                                                            D12 = 1.10  N7 = 1.84665                                                                              ν7 = 23.8                                R13 = 46.19                                                                             D13 = 4.50  N8 = 1.80439                                                                              ν8 = 39.6                                R14 = -15.33                                                                            D14 = Variable                                                      R15 = -25.10                                                                            D15 = 2.50  N9 = 1.80518                                                                              ν9 = 25.4                                R16 = -17.84                                                                            D16 = 1.21                                                          R17 = -17.91                                                                            D17 = 1.30  N10 = 1.69679                                                                             ν10 = 55.5                               R18 = -68.84                                                                            D18 = 2.13                                                          R19 = -29.26                                                                            D19 = 1.40  N11 = 1.77249                                                                             ν11 = 49.6                               R20 = -308.12                                                                 ______________________________________                                        Variable    Focal Length                                                      Separations 28.83        53.97  101.02                                        ______________________________________                                        D2          1.54         1.78   3.05                                          D4          11.51        7.09   0.80                                          D9          4.67         7.19   8.85                                          D11         1.06         2.92   4.42                                          D14         10.66        3.65   0.79                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R10:  K = 4.217    A = 0        B = -1.416 × 10.sup.-5                        C = -1.354 × 10.sup.-7                                                               D = 3.585 × 10.sup.-9                                R14:  K = -2.913   A = 0        B = -8.592 × 10.sup.-5                        C = 1.831 × 10.sup.-7                                                                D = -2.680 × 10.sup.-10                              ______________________________________                                    

NUMERICAL EXAMPLE

    ______________________________________                                        f = 28.87 = 100.85  Fno. = 4.00 - 8.76  2ω = 73.7° -             24.2°                                                                  ______________________________________                                        R1 = 80.90                                                                              D1 = 24.0   N1 = 1.84665                                                                              ν1 = 23.8                                R2 = -245.18                                                                            D2 = Variable                                                       R3 = -53.00                                                                             D3 = 1.20   N2 = 1.69679                                                                              ν2 = 55.5                                R4 = 23.65                                                                              D4 = Variable                                                       R5 = 15.02                                                                              D5 = 1.20   N3 = 1.84665                                                                              ν3 = 23.8                                R6 = 15.55                                                                              D6 = 3.50   N4 = 1.58913                                                                              ν4 = 61.2                                R7 = -22.14                                                                             D7 = 1.00   N5 = 1.84665                                                                              ν5 = 23.8                                R8 = -51.78                                                                             D8 = Variable                                                       R9 = Stop D9 = 3.59                                                           R10 = -20.71                                                                            D10 = 1.20  N6 = 1.80518                                                                              ν6 = 25.4                                R11 = -86.49                                                                            D11 = Variable                                                      R12 = Fixed Stop                                                                        D12 = 2.80                                                          R13 = 3202.97                                                                           D13 = 4.60  N7 = 1.78589                                                                              ν7 = 44.2                                R14 = -15.18                                                                            D14 = Variable                                                      R15 = -26.57                                                                            D15 = 1.20  N8 = 1.77249                                                                              ν8 = 49.6                                R16 = 74.12                                                                             D16 = 2.20  N9 = 1.80518                                                                              ν9 = 25.4                                R17 = 266.07                                                                            D17 = 2.01                                                          R18 = -64.20                                                                            D18 = 1.50  N10 = 1.69679                                                                             ν10 = 55.5                               R19 = -13681.8                                                                ______________________________________                                        Variable    Focal Length                                                      Separations 28.87        54.00  100.85                                        ______________________________________                                        D2          1.93         2.03   3.45                                          D4          8.80         5.94   0.89                                          D8          1.65         4.40   5.54                                          D11         0.73         3.16   5.00                                          D14         12.30        6.16   4.32                                          ______________________________________                                    

The Values of Aspheric Coefficients:

    ______________________________________                                        R4:   K = 2.393 × 10.sup.-1                                                                A = 0        B = -1.146 × 10.sup.-5                        C = 2.167 × 10.sup.-8                                                                D = -2.879 × 10.sup.-10                              R10:  K = 4.558    A = 0        B = -5.356 × 10.sup.-5                        C = -2.028 × 10.sup.-7                                                               D = -2.128 × 10.sup.-9                               R14:  K = -2.794   A = 0        B = -8.249 × 10.sup.-5                        C = 1.650 × 10.sup.-7                                                                D = -2.114 × 10.sup.-10                              ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                      Numerical Example                                               Factor          11      12       13    14                                     ______________________________________                                        |FB/FW|                                                                     0.87    0.87     0.69  0.65                                                   (Gp.5)  (Gp.5)   (Gp.5)                                                                              (Gp.4)                                 |FC/FW|                                                                     0.87    0.87     0.69  0.65                                                   (Gp.5)  (Gp.5)   (Gp.5)                                                                              (Gp.4)                                 F1,2W/FW        1.13    1.24     1.33  1.33                                   FA/FW           0.96    1.01     1.04  1.02                                   νA2-(νA1 + νA2)/2                                                                    46.4    46.4     46.4  46.4                                   (RA3 + RA2)/(RA3 - RA2)                                                                       0.32    0.29     0.31  0.25                                   ______________________________________                                                      Numerical Example                                               Factor          15      16           17                                       ______________________________________                                        |FB/FW|                                                                     0.65    0.94     0.87  0.79                                                   (Gp.4)  (Gp.4)   (Gp.6)                                                                              (Gp.6)                                 |FC/FW|                                                                     0.65    0.87         0.79                                                     (Gp.4)  (Gp.6)       (Gp.6)                                   F1,2W/FW        1.30    1.19         1.32                                     FA/FW           0.99    0.83         0.78                                     νA2 - (νA1 + νA3)/2                                                                  46.4    35.2         37.4                                     (RA3 + RA2)/(RA3 - RA2)                                                                       0.24    0.23         0.17                                     ______________________________________                                    

It will be appreciated from the foregoing that according to the presentembodiment, the entire system is made up from four lens groups and rulesof design for these lens groups are set forth to give proper movingconditions and refractive power arrangements, thus achieving a zoom lenshaving a maximum field angle of about 74° and a zoom ratio of about 3.5with high optical performance throughout the entire zooming range.

What is claimed is:
 1. A zoom lens comprising, from front to rear, a first lens group of negative refractive power, a second lens group of positive refractive power, said second lens group including, from front to rear, a lens unit of positive refractive power, a lens unit of negative refractive power and a lens unit of positive refractive power, and a third lens group of negative refractive power, zooming being performed by varying a separation between each adjacent two of said lens groups, wherein for an object at infinity, letting heights of incidence for a wide-angle end and a telephoto end of an on-axial ray of light on the i-th lens surface, when counted from the object side, be denoted by Hiw (Hiw>0) and Hit (Hit>0), respectively, and letting heights of incidence for the wide-angle end and the telephoto end of an off-axial principal ray of light from the maximum angle of field on the i-th lens surface, when counted from the object side, be denoted by Hbiw and Hbit, respectively, an aspheric surface of such shape that a negative refractive power gets progressively stronger as going from a lens center to a lens margin is applied to a lens surface Ra located on the object side, which lens surface is of negative refractive power and which lens surface is concave toward the object side and which lens surface satisfies the following conditions:

    Hiw>|Hbiw|

    Hit>|Hbit|,

and wherein said zoom lens satisfies the following conditions:

    0.7≦|Fw/f3|≦2.5

    0.2≦|Fw/f1|≦1.0

    1.0≦Fw/f2≦2.5

where Fw is the shortest focal length of the entire system, f3 is the focal length of said third lens group and f1 and f2 are the focal lengths for the wide-angle end of said first lens group and said second lens group, respectively.
 2. A zoom lens according to claim 1, satisfying the following condition:

    0.1≦f3·(1-β3w)/Fw≦0.5

where β3w is an image magnification for the wide-angle end of said third lens group.
 3. A zoom lens according to claim 1, wherein said lens surface Ra is included in said lens unit of negative refractive power.
 4. A zoom lens according to claim 1, wherein said lens surface Ra is included in a negative lens of said lens unit of negative refractive power.
 5. A zoom lens comprising, from front to rear, a first lens group of negative refractive power, a second lens group of positive refractive power, said second lens group including, from front to rear, a lens unit of positive refractive power, a lens unit of negative refractive power and a lens unit of positive refractive power, and a third lens group of negative refractive power, zooming being performed by varying a separation between each adjacent two of said lens groups, wherein for an object at infinity, letting heights of incidence for a wide-angle end and a telephoto end of an on-axial ray of light on the i-th lens surface, when counted from the object side, be denoted by Hiw (Hiw>0) and Hit (Hit>0), respectively, and letting heights of incidence for the wide-angle end and the telephoto end of an off-axial principal ray of light from the maximum angle of field on the i-th lens surface, when counted from the object side, be denoted by Hbiw and Hbit, respectively, an aspheric surface of such shape that a negative refractive power gets progressively stronger as going from a lens center to a lens margin is applied to a lens surface Ra located on the object side, which lens surface is on said lens unit of negative refractive power in said second lens group, which lens surface is concave toward the object side and which lens surface satisfies the following conditions:

    Hiw>|Hbiw|

    Hit>|Hbit|.


6. A zoom lens comprising, from front to rear, a first lens group of negative refractive power, a second lens group of positive refractive power, said second lens group including, from front to rear, a lens unit of positive refractive power, a lens unit of negative refractive power and a lens unit of positive refractive power, and a third lens group of negative refractive power, zooming being performed by varying a separation between each adjacent two of said lens groups, wherein for an object at infinity, letting heights of incidence for a wide-angle end and a telephoto end of an on-axial ray of light on the i-th lens surface, when counted from the object side, be denoted by Hiw (Hiw>0) and Hit (Hit>0), respectively, and letting heights of incidence for the wide-angle end and the telephoto end of an off-axial principal ray of light from the maximum angle of field on the i-th lens surface, when counted from the object side, be denoted by Hbiw and Hbit, respectively, an aspheric surface of such shape that a negative refractive power gets progressively stronger as going from a lens center to a lens margin is applied to a lens surface Ra located on the object side, which lens surface is on a negative lens of said lens unit of negative refractive power in said second lens group, which lens surface is concave toward the object side and which lens surface satisfies the following conditions:

    Hiw>|Hbiw|

    Hit>|Hbit|.


7. A zoom lens comprising, from front to rear, a first lens group of negative refractive power, a second lens group of positive refractive power, and a third lens group of negative refractive power, zooming being performed by varying a separation between each adjacent two of said lens groups, wherein for an object at infinity, letting heights of incidence for a wide-angle end and a telephoto end of an on-axial ray of light on the i-th lens surface, when counted from the object side, be denoted by Hiw (Hiw>0) and Hit (Hit>0), respectively, and letting heights of incidence for the wide-angle end and the telephoto end of an off-axial principal ray of light from the maximum angle of field on the i-th lens surface, when counted from the object side, be denoted by Hbiw and Hbit, respectively, an aspheric surface of such shape that a negative refractive power gets progressively stronger as going from a lens center to a lens margin is applied to a lens surface Ra located on the object side, which lens surface is of negative refractive power and which lens surface is concave toward the object side and which lens surface satisfies the following conditions:

    Hiw>|Hbiw|

    Hit>|Hbit|,

and wherein said zoom lens satisfies the following condition:

    0.1≦f3·(1-β3w)/Fw≦0.5

where f3 is the focal length of said third lens group, Fw is the shortest focal length of the entire system, and β3w is an image magnification for the wide-angle end of said third lens group.
 8. A zoom lens according to claim 7, wherein said second lens group includes, from front to rear, a lens unit of positive refractive power, a lens unit of negative refractive power, and a lens unit of positive refractive power.
 9. A zoom lens according to claim 7, satisfying the following condition:

    0.7≦|Fw/f3|≦2.5.


10. 10. A zoom lens according to claim 7, satisfying the following conditions:

    0.2≦|Fw/f1|≦1.0

    1.0≦Fw/f2≦2.5

where f1 and f2 are the focal lengths for the wide-angle end of said first lens group and said second lens group, respectively.
 11. A zoom lens according to claim 8, wherein said lens surface Ra is included in said lens unit of negative refractive power.
 12. A zoom lens according to claim 8, wherein said lens surface Ra is included in a negative lens of said lens unit of negative refractive power. 